I got a whole slew a different heartcolors, red is the normal hearts, white is the same as in the original (keep it and you get an extra heart on the next floor, or when you pick up a second one), blue hearts are also the same as in the original game. But this time around, there are also black/dark grey hearts. I got one of these as a drop, picked it up and now I have a black heart. Note that this is an extra heart, so it's not a damaged red heart.
Best Answer
They're called Sin Hearts, and they damage all enemies in the room when depleted. Much like the Necronomicon spacebar item.