Gray cats are called ‘blue’ because the gray coat has a faint blue tint. This is because the grey coated cat is genetically a dilute black coat; the effect of the presence of the dilution gene.
The Russian Blue cat breed is gray but the breeder who created the breed decided to use the word ‘blue’ rather than ‘gray’ because cat breeders prefer blue to gray. Why? Because it sounds more interesting and technical. They could have called the breed ‘the Russian Gray’.
You’ll see some blue (gray) British Shorthairs from Russia looking almost blue! Weird (see below). How do they achieve that? Judicious use of vegetable die 😉
A red (ginger) cat with dilution makes cream and you’ll see dilute calico cats (a faint tortie-and-white coat) – see below.
In conclusion blue is gray and cat breeders prefer to use the term ‘blue’ over ‘gray’ as it makes the cat more marketable. That’s my opinion.
See a page on the cat coats affected by the dilution gene: click here.