Blood is ubiquitous in the Doom games and in other games based on the Doom engine (including Heretic, Hexen, Strife, etc), and is perhaps a large factor in making them controversial in some circles (and appealing in others).
The Doom engine creates animated blood spatter when a shootable thing is hit (including the player). Besides enhancing the atmosphere of the game, this helps the player to know that his aim was correct. Blood also splatters when things are crushed. In both cases, Vanilla Doom treats blood as a transient special effect; only the source ports (initially Doom Legacy but later ZDoom) allow for monster blood to splatter onto walls, and stick there.