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Gray and blue are not interchangeable terms and the genetics are DIFFERENT significantly.
Gray is a silver and pg based bird, and is not a BBS based bird so it does not apply.
And no, your possibilities are incorrect since blue to blue can yield, blue, black and splash.
I am pretty sure a blue - you call it gray, rump means you have very dark blues not blacks. My blacks have no blue (lighter) rump feathers. Only my black(with red) lacking melanizers, has a lighter rump and he may be a dark blue with red instead, only breeding will tell his actual genetics, in either case he is not a true black bird.