[quote=MistressCyanide]
I would do some research on it, doesn't blood go blackish anyway if it is out in the air for a while?
[/quote]

That's oxygenation and it turns blood brown (the iron basically rusts), but that takes a while.

Another thing is that if the mask is blood, then he has to focus on it being solid (splitting his concentration) and heat will cause it to turn liquid again, converting him back to Steve. But maybe that's for the best? Give Mr. Vein a weakness so he doesn't get out of control, and also a way to stop him if he does get out of control?

Would it be okay if he can change the color of his blood? I know it's stupid and mundane, with no real applications and absolutely zero logic, but… eh?