IMO, I think the blood is that of the blood of the martyr's that Jesus is returning to avenge them as we read back in Rev 6:9-11.
Rev 6:9 And when he had opened the fifth seal, I saw under the altar the souls of them that were slain for the word of God, and for the testimony which they held:
Rev 6:10 And they cried with a loud voice, saying, How long, O Lord, holy and true, dost thou not judge and avenge our blood on them that dwell on the earth?
Rev 6:11 And white robes were given unto every one of them; and it was said unto them, that they should rest yet for a little season, until their fellowservants also and their brethren, that should be killed as they were, should be fulfilled.
Rev 19:13 And he was clothed with a vesture dipped in blood: and his name is called The Word of God.
Rev 19:15 And out of his mouth goeth a sharp sword, that with it he should smite the nations: and he shall rule them with a rod of iron: and he treadeth the winepress of the fierceness and wrath of Almighty God.
The Lord's Day of Vengeance prophesied in Isaiah
Isa 63:1 Who is this that cometh from Edom, with dyed garments from Bozrah? this that is glorious in his apparel, travelling in the greatness of his strength? I that speak in righteousness, mighty to save.
Isa 63:2 Wherefore art thou red in thine apparel, and thy garments like him that treadeth in the winefat?
Isa 63:3 I have trodden the winepress alone; and of the people there was none with me: for I will tread them in mine anger, and trample them in my fury; and their blood shall be sprinkled upon my garments, and I will stain all my raiment.
Isa 63:4 For the day of vengeance is in mine heart, and the year of my redeemed is come.