Rev 19:11 Then I saw heaven opened and here came a white horse! The one riding it was called “Faithful” and “True,” and with justice he judges and goes to war. 19:12 His eyes are like a fiery flame and there are many diadem crowns on his head. He has a name written that no one knows except himself. 19:13 He is dressed in clothing dipped in blood, and he is called the Word of God.

Whose blood?

(a) The blood of the martyrs?

So Caird.  But, as Koester remarks:

‘Christ’s blood makes the robes of others white, rather than their blood making his robe red (Rev 7:14).’

(b) The blood of God’s enemies?

Ian Paul considers that the image is that of God trampling the nations in the winepress of his wrath, with their blood spattering his garments (borrowing from Isa 63:1-4).  So also Charles, Patterson, Michaels, Beale, Osborne, Roloff, Fee.

Mounce:

‘The blood that stains the robe of the conquering Messiah is not his own (as in Rev 1:5; 5:9; 7:14; 12:11), but the blood of the enemy shed in conflict.’

Ladd:

‘The picture here is of Christ the warrior and conqueror of evil, not of Christ the redeemer.’

This interpretation is defended in some detail here.

(c) Christ’s own atoning blood?

Gorman explains that this is:

‘because the battle has already been fought and won in his death’

Origen:

‘John does not see the Word of God mounted on a horse naked. He is clothed with a garment sprinkled with blood, since the Word who became flesh, and died because he became flesh, is invested with traces of that passion, since his blood also was poured forth upon the earth when the soldier pierced his side.’

Oecumenius:

‘He says, Clad in a robe sprinkled with blood: for even in the vision the Lord was bearing the marks of his passion, and was showing his all-holy body all but covered with his precious blood.’ (Quoted by Patterson)

Morris:

‘This is surely a reference to Calvary; Christ overcame by shedding his blood…It is more than difficult to hold that John writes of blood without a thought of the blood shed on the cross. In this book he repeatedly makes the point that it is in his capacity as the ‘Lamb as though slain’ that Christ conquers. He overcame, not by shedding the blood of others, but by shedding his own. Besides, at this point the battle has not yet been joined (not until vv. 19–20), nor the winepress trodden. Moreover his sword is the Word (v. 15). John is surely saying “In the power of suffering love, Christ rides forth conquering” (Stoffel)’

According to EBC:

‘The imagery in this verse has traditionally been related to Isa 63:1–6, a passage understood messianically by the Jews and one that John has used in portraying God’s wrath in Rev 14:9–11, 17–19. Isaiah pictures a mighty warrior-Messiah who slaughters his enemies. Their life-blood splashes on his clothing as he tramples them down in his anger, just as the juice of grapes splashes on the winetreader in the winepress. But is Christ’s blood-dipped robe red from his enemies’ blood or from his own blood? There are good reasons for accepting the latter. If the blood is his enemies’, how is it that Christ comes from heaven with his robe already dipped in blood before any battle is mentioned? Moreover, the blood mentioned in connection with Christ in the Apocalypse is always his own life-blood (Rev 1:5; 5:6, 9; 7:14; 12:11). The word “dipped” does not fit the imagery of Isa 63:2; but it does fit that used in Revelation of believers’ garments being washed thoroughly in Christ’s blood (Rev 7:14; 22:14). Finally, the sword with which Christ strikes down the nations comes from his mouth and is not in his hand (v.15); this too is incompatible with battle imagery.’

Wall notes that:

• The robe is already stained with blood before the battle begins;
• It should be understood with reference to Rev 5:5f (rather than Isa 63:1-6);
• It is consistent with the overall Christology of Revelation to regard the conquering Lamb as the slaughtered Lamb.

Wall concludes:

‘On his robe are the stains of his passion, because of which he is called the “Faithful and True” to God’s love and by which a people has been purchased for God.’

Mulholland notes that in John’s vison the nature of garment worn relates to the nature of the person wearing them.  Therefore, we must presume that this is the warer’s own blood.  Contra Beale, who thinks that the blood-stained garments symbolize God’s retributive justice, Mulholland says that God’s judgment has already been satisfied at the cross.

Koester (AB):

‘Revelation says that Jesus’ blood (haima) advances God’s kingdom by delivering people from sin (Rev 1:5; 5:9). Jesus’ blood makes the robes of the redeemed white (Rev 7:14), like the robes of those who follow him into battle. His blood also brings victory over evil (Rev 12:11), and here he defeats the satanic beast and false prophet. Since Christ appears in a bloodstained robe before the battle begins, the blood must be his own.’

Koester adds, as one of his reasons for rejecting interpretation (b):

‘Revelation transforms older images in light of Jesus’ death and resurrection. In Rev 5:5–6, John invoked the image of the lionlike ruler from Judah from Gen 49:11.  [But] he showed that the promise of the conquering Lion was realized in the slain Lamb. The pattern in Rev 19:13 is similar. The vision of the divine warrior has similarities to Isa 63:1–3, but the imagery is recast so that Christ comes to the battle in robes stained with his own blood, since his sacrificial death has been his path to victory.’

Conclusion

I am inclined to think that this expression refers to Christ’s own atoning blood (option c).