The question about render texture

Hello everyone , sorry for my bad english in advance.
I have a question to ask . I tried to make a render texture and set a semi_transparency texure gameobject in front of the camera that dealed with RT.
But I have no any idea that how to get rid of the black color or solid color of background ,when I want to use blendmode in the gameobject .
I’m sure I set the render texture include alpha , but I don’t want that black or solid color of background being added with src color , only src color and alpha finally is rendered.
which combination of blendmode can solve my problem?
I also tried add material into the render texture ,but it seems to be not a point ?
I use built-in.