Only noticed after that whole scene already played out, but just rifling through the stuff at his tent reveals what is referred to in text as an empty blood jar, and there's a spattered bowl off to the side on a rock.
The huge mirror, sure, I just assumed it was a humorous nod at his vanity. But seriously? How does the entire party miss all of this? He'd have to sparkle to be more obvious.
Actually, no, Astarion would 100% wear body glitter or at least get someone else's on him.