Hello,
I have object A and object B. Object B has a shiny metallic surface.
I want both objects (A and B) to be visible in the scene. I also want Object B to reflect everything in the scene except for object A.
Is it possible to control the visibility of object A in the mirror surface of object B so I could choose to have it visible, invisible, or even a little bit visible?
In Eevee, it’s shockingly easy- just turn Screen Space Reflections off:
In Cycles, it takes quite a bit more work, but it is actually possible:
This doesn’t work with emissive objects, but the object itself is still invisible, there’s just obviously something missing in the reflection:
I will not be using emissive objects but I will be using Cycles. I am waiting for a hefty animation to render but as soon as it has, I’ll be on this. Thank you so much.
Right then. I followed your lead. Here is what I have before touching the shaders.
This is what I do to the red cube:
And this is what I do to the blue cube:
And here is the result:
I have just followed what you did, but there is still one remaining small question. Instead of the blue cube being invisible [0] in the mirror, how can I have a visibility in the mirror that is some value between 0 and 1? The blue cube should stay unchanged outside the reflection.
I don’t think you can- you can’t really have a halfway point between “is this object visible to X light ray or not”. That being said, you can kind of hack it like this:
A Translucent BSDF works slightly better at very low visibility:
You could do it like this:
Edit
You can even cut holes in the reflection 
:
Note the gradient in the first hole.
This is perfect, thank you. It is also very useful when reflections get too complicated and so distracting.
I’ll try not to look at your solution for holes in the reflection!!! 
