easiest shader ever.
To apply screen shader to 3d You need to child some ColorRect to root node and extend it to whole screen ( remember to set mouse filter to ignore )
You add material and new shader to ColorRect.
uniform float switch
fragment() do :
COLOR = mix(vec4(0.0),vec4(1.0,0.0,0.0,1.0),switch)
and now You can turn your blood flash on with
which You should switch between player being hit and some timer countdown after hit
If You don't know about shaders yet, just check the documentation and my code will become clear to You