Are you sure the shower is going to the black directly? There was another thread on here a little while back about somebody encountering a grey tank that T'd into their black tank behind the valve instead of in front of it. (No, I don't recall what model that was.)
But, if you've got some other kind of crazy tie-in like that, you might be draining to the grey, but then have the grey draining to the black resulting in the black being full.
Either way, it would still need to be fixed.
There's no use crying over spilled milk, unless it's on your keyboard.