Blood loss is one of the Afflictions that can afflict the player during The Long Dark. It is typically caused by being attacked by Wolves or a Bear, but the player can also obtain it by not treating a Laceration.
Blood loss simply causes a straight loss of Condition. There will also be a pool of blood where the player was attacked. This pool will go away in a few days and may not be removed in any fashion. It can attract predators.
Blood loss may not be treated. Provided the loss of Condition doesn't kill the player, the effect goes away immediately.