There are a few types of Blood Stains within the game:
Black Blood Stains: Like Demon's Souls, these dark black Blood Stains show the last few moments of a player who perished at that spot. You can click on 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: These blood stains are left by player deaths in your world, either your own or an invading black phantom. These contain any lost humanity that the player or invading phantom contained plus all of the souls that the player or invading phantom 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.