Bloodstains grants an equip unique bonuses during battle and attribute boosts when equipped.
There are two ways to assign a Bloodstain to a weapon. Please note that like most features here, this ability is hidden when you first start the game and unlocked later on.
Staining a weapon consumes 100 blood drops, and using too much may decrease your Blood Diary progress and/or decreases respect Mercenaries have for you.
The Bloodstain list can be accessed by going to "Information>Bloodstains".