Aren't you setting the color of your decal to red?
decal:SetColor(1,0,0,1)
Looking at the Projectile.Lua script
local decal = Decal:Create(mtl)
decal:AlignToVector(pickinfo.normal,2)
decal:Turn(0,0,Math:Random(0,360))
decal:SetScript("Scripts/Objects/Effects/BulletMark.lua")
if mtl~=nil then mtl:Release() end
decal:SetPosition(pickinfo.position)
decal:SetParent(pickinfo.entity)
You might like to add decal:Turn line to your script so that the blood looks more natural.
Hope this helps.