Wiki: env_blood (Half-Life)

env_blood - Point Entity

When triggered, an env_blood entity will spawn a spray of blood, similar to the blood effect generated when one attacks an enemy.

Name (targetname) - Name is a general attribute used to identify entities.
Pitch Yaw Roll (Y Z X) (angles) - Sets the direction the spray will go (only used by certain entity flags).
Blood Color (color) - Sets the colour of the blood.
- Red (Human) (0) - Seems to be greyscale, despite the name.
- Yellow (Alien) (1) - The yellow colour of blood from aliens like monster_headcrab and monster_houndeye.
- (2) - The red colour of blood from humans like monster_scientist and monster_human_grunt.
Amount of blood (damage to simulate) (amount) - Sets the scale of the blood spray. Defaults to 100. Smaller values will create a smaller spray and higher values will create a bigger spray.

Random Direction (1) - If enabled, the spray will pick a random direction rather than use Pitch Yaw Roll.
Blood Stream (2) - If enabled, the spray will be a stream of blood that shoots out in the direction specified in Pitch Yaw Roll. The distance of the spray is affected by the Amount of blood attribute. If not enabled, the spray will be a directionless splash.
On Player (4) - If enabled, the blood spray will be created on the player (as though the player is being hurt).
Spray decals (8) - If enabled, the spray will also create a blood decal in the direction specified in Pitch Yaw Roll when the spray is triggered.

The grey coloured spray can be used to simulate dripping water. The effect may not look ideal for all situations, though.