Decal problem

So I create a decal texture to use for a map I'm working on, and it's meant to sort of meld with the ground texture, but when I place it, it has this weird dark outline around it both in the editor and in-game
If I modify the texture to be opaque and give it a background that matches the ground, then the dark line appears on the edge of the background instead
I am unsure how to fix this, or if it would even be possible to fix, but any insight would be much appreciated

Well that's discouraging