Alpha Error

Hello,I have a camera to render the game and another to render the UI.This cameras have a render texture,each one is drawn in a rect that is render by another camera.I have some transparency problems,like I show in the next image.In the right side of the image we can see the expected result, in the left side can we see the unexpected result.The text has alpha but the background blocks the visibility to the game just in the back,however for some reason,this is shown.Why?Please help.

 

I have build a simple example to understand better my problem.

The red cube is rendering in one camera and it is back of the green texture, the green texture and the blue texture with transparency in another.The blue texture is in front of the green texture( 3 elements on the right ).
Over the top,I can see in the Game View, the red cube although the green texture is opaque.How I Solve this problem?

 

Fixed with a blending operation in the alfa texture.