Is there anything special about the blood rooms?
Solution 1:
If you damage yourself some inconsistent number of times on the spikes in the room, a chest appears.
Solution 2:
They are great for the nun power-up, you can get free re-rolls.
Solution 3:
Try using the pony power, or the book that gives you a shield, and walk over it. A chest will appear, and you won't take any damage.