I have an issue using the Decal actor, I have made a decal material which works with the actor. However when I hook up the Normal input (I have tried both a texture sample and a vector3 set to 0,0,1) The decal turns black.
I have also tried using the DBuffer system however it produces the same results example here
From the Picture, it looks like your Decal is backwards. I am making that judgement based on the decal arrow which instead of pointing to the wall is pointed out from the wall. This could cause errors with the normal which might present with black. If you are trying a DBuffer Decal, make sure you have added the (r.EarlyZPassMovable = 1 r.EarlyZPass = 2 r.DBuffer = 1) to the DefaultEngine.ini in the project config files or into the BaseEngine.ini in the Engine config files under [SystemSettings].