yeah.. that's what just about every mtg player I know calls it: Monocolor. Shape names are great, but if a particular name doesn't really add anything to the design process, it isn't that useful. I'd stick with the "industry standard" in this case.
I kind of agree with 'closed' in the sense of a closed set, but most people would probably recognize what is meant if the word 'mono' was used. In a sense a mono-archetype is already the establihed term for this, I feel?