I didn't feel comfortable promoting people to blue until I got to brown belt, plus I had a black belt's blessing (my instructor is a 3rd degree now, I think, so must have been 2nd degree when I started promoting people to blue).

Anything above that needs to be directly from a black belt, for my comfort level at least. Though I could see a brown promoting somebody to purple in specific situations, like the black belt they usually ask to grade people was unable to make it to the club for a long time.