UGUI Rendering Sprite edges into Alpha Channel

I am attempting to overlay a rendertexture from a secondary UI camera which only renders UGUI elements.

The main camera has a script which blits the rendertexture that the UI camera creates over itself.

My problem is that around the edges of sprites it is rendering some dark edges. I've traced the issue down to the UI elements rendering their edges into the Alpha Channel.

I've attached a screenshot showing this, as well as the script and shader I am using.

I need to use a render texture to do the overlay. Any advice?

 

Anyone?