A) No, I doubt whether the bird you spotted is a hybrid with a blackgrouse. I can find no records of hybrids between these two grouse species, though it is true that blackgrouse will hybridise with capercaillie. The offspring are known as racklehens. I once saw two racklehens at a blackgrouse lek in Finland. While one was a good-looking bird, bearing the features of both parents, the other was an ugly individual.
Red grouse have plumage that can be highly variable, with no two birds looking exactly the same as each other. Some have white spots, others may be blackish, but entirely black grouse, such as the one you describe, are rarely encountered.