Russian Blues are a ‘natural’ breed, meaning that they weren’t created by breeding random blue cats together; but the cats already existed. Breeders then took them (from the Archangel Isles) and selectively refined and defined the breed for distinguishing traits.
The breed is exclusive of any other breed of cat excepting in the early 1900s. There was a brief stint after World War II when the gene pool was replenished by outcrosses with blue-point Siamese and English Blue Short-hair.
Because of the selectiveness and carefulness, the breed has a whole consistent look. Each mating of Russian Blues produces a litter of Russian Blues.