Drawing Black to Render Target with additive blend material: HELP

Hey guys,
I’m having a issue drawing black/erasing a render target canvas. I currently have a material that is driven vector parameter, that is being fed by a line trace to get the UV coordinate, that draws a circle to the render target as my character moves along the plane

. This is for a Fog of War Opacity mask. It works perfect for adding the fog of war but I am trying to reverse the fog of war once a certain time elapses. I can’t get it to erase in a additive way. It seems to not let me draw black onto the render target. Can someone please help/shed some light. Thank you. Here is the material to draw onto the render target.