He sees more of Strongclaw's blood than hers. Of course, he's fresh out of a fight, and Sablefrost hasn't unsheathed her claws for months. But then they're fighting, and it's hard to tell whose blood is whose, or if it makes any difference at all.
Sablefrost is winning and he lets himself hope. Thinks that this one small remnant of Embertooth will live. Strongclaw kicks her into the air and he lets that hope die
Thornstreak wasn't meant to get what he wants.
The 'fluid' in this drabble is blood. I know it didn't say it, but it would've turned out all 'cliché-trying-to-find-a-synonym-for-blood'.