There are a few types of Blood Stains within the game.
Dark Blood Stains
Like Demon's Souls, these dark Blood Stains show the last few moments of a player who perished at that spot. You can examine the stain in order to observe a phantom of that player reliving their death. If you see one, be wary of traps, falls, and difficult enemies.
Glowing Green Blood Stain
This blood stain is left after your death in your world. It will be on the spot where you died. If you died by fall damage or by falling down chasms, the blood stain will be on the spot ~5 seconds before the time of your death. This contains any loose Humanity that the player contained plus all of the souls that the player had on them at the time of death. A second player death will cause the previous blood stain and its accumulated souls to disappear and a new blood stain with the number of souls and Humanity contained at the time of that second death to appear.
Using the Darksign to kill yourself doesn't leave any blood stain nor erase any previous blood stain.
Glowing Red Blood Stain
Upon the death of an invading Red Phantom, it will leave its own Blood Stain. It's basically the same as the green version. It contains the Humanities and Souls the invading phantom carries before its death.
NPC Dark Spirits also leave this upon their deaths.