Or it is literal, that the souls of the dead can speak because they are alive. Souls of the dead speaking is not comparable to blood speaking since blood cannot speak literally. Not only did the dead speak, but God answered and spoke back!
We all know that the soul can speak...but we are talking about dead people here under an altar. We must not take just one verse out of a symbolic apocalyptic book and base an entire genre of doctrine upon it. Let's look at the context and the traditional historicist hermeneutic of the seals, which deal with the history of the church and it's slow but steady slide into apostasy, while God preserves a remnant
6:1-2 The first four seals are widely known as the “Four Horsemen of the Apocalypse.” In Revelation the horse is an animal associated with war. See Revelation 19:11, 19. The four horsemen represent the churches as they go forth to battle their enemies. The colors of the horses (white, red, black, and pale) indicate a change from purity to apostasy.
The 1st Seal: A White horse: The Pure Church
White represents purity and righteousness. Revelation 7:14; 19:8; Isaiah 1:18. The pure white church went out “conquering and to conquer.” During this time the Gospel spread like wild fire. Paul said that the Gospel “was preached to every creature which is under heaven” (Colossians 1:23), and his opponents said that they had, “turned the world upside down.” Acts 17:6. The white horse is the apostolic church going forth victoriously under the banner of Prince Jesus. The 1st seal represents the same period as the 1st church - 0-100 AD.
6:3-4 The 2nd Seal: A Red Horse: The Persecuting Church. Red is the color of sin (Isaiah 1:18), the dragon (Revelation 12:3) and bloodshed or sacrifice (2 Kings 3:22-23). When the faith and devotion of the Church became corrupted it lost the power of God to carry the Gospel forward in victory. It then began using the power of the state, uniting with the dragon (Pagan Rome controlled by Satan) to persecute dissenters. The phrase “kill one another” talks about the persecution during this era. The great sword that the horseman carries represents this war and bloodshed. (Isaiah 3:25, 1 Chronicles 22:8, Acts 12:1,2) The 2nd seal represents the period 100-323 AD just as the 2nd church did ending with the conversion of Constantine. It also depicts the compromise that began in this period when errors arose and worldliness came into the church. During this period church and state began to work together.
6:5-6 The 3rd Seal: A Black Horse: The Corrupt Church. Black is the color of darkness, apostasy and error (Acts 26:18, John 12:35), the very opposite of white. The “balances” used for measuring “wheat” and “barley” are a symbol of the buying and selling of the Gospel, salvation and forgiveness. The Church, during this time, entered a period of great moral and doctrinal error, substituting pagan beliefs and practices for the truths of God’s Word, and merchandising the Gospel. Thus they caused a scarcity of the bread of life and made commerce of it. The Bible was a banned book during this period, and preaching from the Word of God was hardly heard. There was indeed a famine for the Word of God. (Amos 8:11,12) God decreed, “Hurt not the oil and the wine,” which symbolize the Holy Spirit (Zechariah 4:3, 6) and the blood of Jesus (Matthew 26:26-29). Even though the Church was in great darkness, God preserved a people with the true Gospel of salvation.
The 3rd seal represents the same period as the 3rd church - 323-538 AD and was filled with superstition and darkest errors.
The balances in the hand of the black horse represent union of church and state. William Miller states:
The balances denoted that religion and civil power would be united in the person who would administer the executive power in the government, and that he would claim the judicial authority both in church and state. This was true amongst the Roman emperors from the days of Constantine until the reign of Justinian, when he gave the same judicial power to the bishop of Rome. i/
6:7-8 The 4th Seal: A Pale Horse: The Dead Church. This pale horse is the color of nausea and death. The Church that should have been pointing the way to eternal life was instead spreading death. “And Hell [the grave] followed with him.” The spiritual “Death” that spread across Christendom was accompanied by a moral and intellectual paralysis. The Holy Scriptures were forbidden to the people; nobles, commoners and clergy alike were largely ignorant of the great truths they contain. For centuries the Christian world made no progress in science, arts or civilization. Millions lived miserable lives of ignorance and squalor.
Ezekiel 14:21 lists war, famine, pestilence, and wild beasts as judgments on apostasy from God. Whenever the Church takes up the sword to coerce the conscience, a famine of God’s Word results. Pure Bible truth is replaced with the pestilence of heresy, opening the way for persecutors (wild beasts) to punish and chastise the Church, and multitudes perish. This was the time referred to in Revelation 12 where the woman (God's Church was nourished in the wilderness (in hiding) and covers the period when the Papacy bore its unrebuked, unrestrained and persecuting rule. The fourth part of the earth refers to the territory over which this power had jurisdiction. The verse says this period had power to kill with "sword", "hunger", "death", and the "beasts of the earth". This is exactly how the Papacy killed God's people.
The period of this church stretches from 538 AD to the time of the Reformers, approximately 1450 AD.
6:9-11 The 5th Seal: The Martyred Church. “Under the altar the souls…slain for the word of God.” Apocalyptic language teaches lessons and truths in pictures or symbols. God does not keep literal souls under a literal altar in heaven. The time covered by this seal follows the period of papal persecution of the fourth seal, and therefore it would begin when the Reformation began to undermine the papal power and restrain the persecuting power of the Roman Catholic Church. The Reformation officially began in 1517 with Martin Luther nailing his 95 theses to the Wittenberg Church door, however, prior to Martin Luther, Wycliffe, Huss, Jerome and others began writing to reform the Catholic Church and many of the points Luther raised had already been raised by them. The starting point, therefore, of this seal is approximate.
During the middle ages, the Papal system had millions of people put to death because they would not renounce Bible truth. Here is pictured, in symbolic language, the many martyrs of the preceding seal, whose blood, like the blood of Abel (Genesis 4:8-10), cries out to God for justice and a vindication of the truths for which they died. This crying out for justice is an example of personification used in the Bible. Personification is used in other areas of the Bible: Habakkuk 2:11, James 5:4
Although some might wish to use this verse as proof that spirits are in heaven in a conscious state, the Bible teaches that the dead in Christ rise and put on immortality when He returns. Until such time, they are asleep. 1 Corinthians 15:51-53. Heaven would not be a very pleasant place if there were souls shut up under an altar crying.
The altar mentioned is symbol of sacrifice (specifically papal sacrifice). Adam Clarke writes:
A symbolical vision was exhibited, in which he [John] saw an altar; and under it the souls of these who had been slain for the word of God--martyred for their attachment to Christianity--are represented as being newly slain as victims to idolatry and superstition. The altar is upon earth, not in heaven.
They had gone down to the grave in the most ignominious manner. Their lives had been misrepresented, their reputation tarnished, their names defamed, their motives maligned, and their graves covered with shame and reproach, as containing the dishonored dust of the most vile and despicable characters. Thus the church of Rome, which then molded the sentiment of the principal nations of the earth, spared no pains to make her victims an abhorrence to all people. i.
In verse 11 they are given white robes. After the Reformation began its work, those that had died were vindicated - it was seen that they had suffered, not for being vile criminals, but for "the word of God and for the testimony which they held".
Then their praises were sung, their virtues admired, their fortitude applauded, their names honored, and their memories cherished. White robes were thus given to every one of them. ii.
This seal just as the others represents a period of time, and this one begins when the Reformation began to undermine the papal dominion and restrained the persecuting power of the Roman Catholic Church.
"A little season": The cruel work of the Roman Catholic church did not cease even after the Protestant Reformation had been widely established. It continued until in the 1750's when finally the spirit of persecution was restrained. The full vindication of their cause would have to be delayed.
"until their fellowservants also and their brethren, that should be killed as they were, should be fulfilled" refers to the multitudes that were still to be persecuted during the Reformation. And the reformation is not yet over. Many reformers gave up their lives for their work of bringing the Bible to the people and speaking against the errors of the Roman Catholic faith. Nicholas Ridley, Hugh Latimer, Lawrence Saunders, and many more were killed for their stance against the Catholic church.
i. Commentary on the New Testament, Adam Clarke, Vol I, p. 994