The blood mechanic does not always work. A lot of the time you can be practically showering in the blood coming down off of enemies or bosses and getting zero health for it. The guardian of hell is really really bad for this and you basically have to be hugging their ankles point blank to get any health even when the blood clearly spraying all over you. This is really bad because the visuals make you misjudge where the health zone you need to be in actually is.