Mentioned in Night Watch and Thud!: After Sam Vimes goes through a bunch of enemies, he ends up covered in blood. In Night Watch (involving Time Travel), he tells Coates he's from another time (the future). Coates takes a look at the blood-covered guy in armor Dual Wielding swords and asks "From how far back?" In Thud, the guards run at the sight of him.
The Chronicles of Amber by Roger Zelazny: In The Guns of Avalon, after the climactic battle Corwin is soaked in blood. One of his colleagues tells him he's wounded. He tells them no, he's just tired; the blood is that of his brother. He didn't kill him, though—he got there just as the man was dying.
Sharpe: Harper poses as a wounded French soldier to get the Chosen Men into a fort (the blood comes from pulling the rotten tooth that's been bothering him all episode). The unfortunate Frenchman who happens to be passing right when the signal is given and gets subjected to a huge man covered in blood leaping up, grabbing him and growling in his face looks likehe needs new pants.
The samurai in Legend of the Five Rings live according to a strict code of honor, and part of that honor code involves following rules about personal cleanliness. A samurai does not present themselves as dirty or touch corpses, for example. However, it also mentions that only applies in peacetime, and when a samurai is on the battlefield those rules about cleanliness are suspended. As it explains, a samurai who walks off the battlefield covered in the blood of her enemies is hardly someone you would call dishonorable. A samurai is still expected to ritually clean themselves and their equipment once the time for killing is over though.
Mount & Blade has this for both people who have sustained injuries or dealt them. After a battle, especially one where the player has a large bladed weapon as their primary equipment, there's a fine crimson layer on most player and horse models by that point.
Due to the way Team Fortress 2 handles blood decals, they only appear on a player when they are shot by any projectile that isn't an explosive or fire-based, as well as melee attacks. It also has zero indication on actual healthiness because a medic could be healing that heavy, meaning he could look like he's covered in his own blood, yet has well over his normal allotment of 300 HP.
God of War 3 even goes as far as to have an achievement/trophy for having had a given number of gallons of blood coating Kratos.
Grunt in Mass Effect 3 after surviving a wave of rachni husks, provided you secured his loyalty in the previous game.
Samurai Jack: but with oil instead of actual blood, because it's a kid's show.
Legend has it that the Austrian flag's creation is related to this. Arch-Duke Leopold V of the Österreich was soaked in blood from a battle, and took off his belt - under which his clothes were still white. Apparently he liked that sight well enough to make it the (new) flag of his Duchy.
After the Battle of Lepanto Genoan Admiral Andrea Doria was covered with the gore from a round shot which had caught an unlucky sailor next to him. In a subversion he had spent most of the battle harrassing the Ottoman flank rather then in the thickest part and the blood spattering was a fluke.