Typically, only special rooms are marked on the map (item rooms, shops, arcades, bosses, deadly sins, etc.).
What's special about the blood room shown below? I tried hurting myself on the spikes, and nothing happened.
The sacrifice rooms will drop chests if you take enough damage, but if you use items that give mercy invincibility, like iv bag or whore of Babylon. Also, if you have tarot card The Chariot, A pony, White Pony, Book of Shadows, or Numerous other invincibility Items, you will get a chest for free. If you have the Left Hand trinket ( WoTL ) the chest will be a Red Chest.