How to control how transparent a Diffuse BSDF is when using EEVEE's Blend Mode? Sorry for the vague title. I'm not sure if I'm asking the right question.

This cube has two materials slots, the faces facing the camera are completely red, and the back faces are completely blue.





When "Blend Mode" is Opaque on both materials, it looks red, just like expected:



(To simplify the problem, I removed the lights from the scene so it's only lightened up by the ambient.)

When the red material's "Blend Mode" is set to Alpha Blend, nothing really changes:




But when both materials' "Blend Mode" is set to Alpha Blend, the red faces become invisible!




So my question is, why doesn't it blend into some kind of purple? And how to control the transparency of these Diffuse shaders so they can blend together, like 10% red 90% blue?
You can keep the alpha blend setting on the red material. You just need to add a mix shader and a transparent shader set to pure white :



The factor of the mix shader drives how much the red blends with the blue.