Transparency blend in widget images

I have a widget that contains an image of a straight line and an image of a square filled with a transparant fill. If I draw the square the line, the line is visible through the transparent fill of the square. I would like the transparency of the square to go over the line, so that if the square is over the line, you can see through that part of the line. Is there something such as some sort of blend mode for widget images that would make this possible?

The images are currently drawn in the widget by using materials. I can find blend modes for textures within materials, but I can not find a way to change the blend mode of (UI) materials with other (UI) material.

Some pictures:

What it looks like now:


What I want it to look like:


Thanks in advance.