Draw Material to Render Target - Too Dark at the edge

Hey,

I try to draw an unlit, translucent material to a render target ( in a blueprint ) and get really bad grey & black spots and borders around the ( cyan ) brush:

Looks like the color values are mixed wrong and get too dark at the semi transparent outside of the brush.

I use this simple brush setup:

How can I fix the dark spots at the edge area of the brush ?