I just started playing the Binding of Isaac and one of the pick-ups I came across was a black/blue-ish heart (much like the red ones that fill up your HP). Initially, I thought it was a sort of "heart container" that increases my overall HP, but I noticed that I couldn't pick up additional health to fill it up.
Here's me after picking up the black heart:
Then, after taking some damage, I noticed that the extra heart was gone (I was back to the three red hearts alone). So I was wondering, do black hearts simply serve as a temporary HP boost, or are they more complicated than that?