I would suggest that Rev 6:9 alludes to several OT texts with both verbal allusions and conceptual allusions. First, let me quote Rev 6:9, 10 -
9 When the Lamb broke the fifth seal, I saw underneath the altar the souls of those who had been killed because of the word
of God, and because of the testimony which they had maintained; 10
and they cried out with a loud voice, saying, “How long, O Lord,
holy and true, will You refrain from judging and avenging our
blood on those who live on the earth?”
The reference to something at the base of the altar or beneath the altar is referenced a few times in the OT:
- Lev 4:7 - The priest must then put some of the blood on the horns of the altar of fragrant incense that is before the LORD in the Tent of Meeting. And he is to pour out the rest of the bull’s blood at the base of the altar of burnt offering at the entrance to the Tent of Meeting.
- Lev 4:18 - The priest must then put some of the blood on the horns of the altar of fragrant incense that is before the LORD in the Tent of Meeting. And he is to pour out the rest of the bull’s blood at the base of the altar of burnt offering at the entrance to the Tent of Meeting.
- Lev 4:25 - The priest must then put some of the blood on the horns of the altar of fragrant incense that is before the LORD in the Tent of Meeting. And he is to pour out the rest of the bull’s blood at the base of the altar of burnt offering at the entrance to the Tent of Meeting.
- Lev 4:30 - Then the priest is to take some of its blood with his finger, put it on the horns of the altar of burnt offering, and pour out the rest of the blood at the base of the altar.
- Lev 4:34 - Then the priest is to take some of the blood of the sin offering with his finger, put it on the horns of the altar of burnt offering, and pour out the rest of its blood at the base of the altar.
There are many more in places like Ex 29:12, Lev 8:15, 9:9, Deut 12:27, Eze 43:13, etc.
Then there is the "How long" allusions, all asking how long God will continue to appear to remain inactive in rescuing or judging wickedness such as:
- Ps 94:3 - How long will the wicked, O LORD, how long will the wicked exult?
- Ps 89:46 - How long, O LORD? Will You hide Yourself forever? Will Your wrath keep burning like fire?
- Dan 8:13 - Then I heard a holy one speaking, and another holy one said to him, “How long until the fulfillment of the vision of the daily sacrifice, the rebellion that causes desolation, and the surrender of the sanctuary and of the host to be trampled?”
Then there is the cry for justice, particularly that of the blood:
- Gen 4:10 - What have you done?” replied the LORD. “The voice of your brother’s blood cries out to Me from the ground.
Note the totality of the picture painted by the text of Rev 6:9, 10 - how long, Sovereign Lord are you not judging our blood and avenging our martyrdom? It was the martyrs' blood that was symbolically pored out at the base of the altar.