How do black hearts function

You can unlock Judas by first beating Mom, which will unlock two new dungeon levels called "The Womb". Then you have to beat The Womb, by killing the boss Mom's heart.

I have just managed to kill Mom, after a few previous attempts that did not go so well. What made it possible for me were two things, basically:

• High health: I had 7 full hearts when I faced mom. This was partially due to luck (lots of extra health pickups/pills) and also due to me hoarding money (which allowed me to fill up on health from the shop and arcade on the same level before the final showdown). I beat mom with at least 2 hearts to spare, though (possibly 3... I was too busy not dying to pay proper attention).
• High damage: I was lucky enough to have some decent damage by the time I faced mom by picking up a small rock, and by clinging to a strength tarot card until that very showdown. This allowed me to empty Mom's health bar quickly, and was essential in handling the lesser baddies that Mom keeps spawning (I also picked up the common cold for some poison damage over time, which was quite handy).

Of course, this would not have helped me without at least a bit of strategy:

• Clear out the stage. Mom's stage usually contains an assortment of rocks. Try to get rid of them as fast as you can, as all they do is limit your maneuverability (also, they might hide a handy power-up, rarely). Of course, if it's not getting hurt or clearing out some rocks, leave the rocks alone. Be aware that Mom's stomp also destroys rocks.
• Always be mindful of the shadow indicating Mom's incoming stomp. At the very least get out of the way, but if you really want to make your life easier try to lure troublesome enemies into the damage area. I haven't seen a spawned baddy that survived a single stomp.
• If you have to stay still, do it in one of the corners or at the center of the room. Mom can damage you directly if you stand too close to one of the four doors on her stage, and this will also let you keep an initial distance from spawning baddies (which spawn from the doors). Actually, try not to spend too much time in front of the doors regardless, even while running around.
• If there's nothing trying to kill you, hug a wall, pick a door and shoot at it. Most chances are one of Mom's body parts will poke through one or more of the doors soon enough, and there's no reason not to cash in on that for some heavy damage.

I managed to pull this off without using my item at all (I had Mom's pad). I suspect that if I were to have a useful item (such as my little unicorn), the battle would have been even easier.

Also, there is an item called the Bible that makes the fight against Mom amazingly easy, as it instantly kills her when used (and it instantly kills the real final boss as well!).