A blue cat will have a blue / slate tinge to the coat. You folks are hoisting yourselves on your own petards by admitting the fact...
Also, just to prove a point, and you guys can look it up for yourselves:
The genotype of a standard grey or brown tabby is: A-B-C-D-iiT-/tbtb (the difference is in the polygenes)
The genotype of a blue tabby is: A-B-C-ddiiT-/tbtb
The genotype of a solid blue cat is: aaB-C-dd
The genotype of a blue & white cat is: aaB-C-S-dd
So any cat with the dd gene is blue irrespective of whether tabby or not. Simple and easy to remember. The D gene will either give you a black cat (aa i.e. non-agouti) or a grey / brown tabby (A agouti)