Well, if you look at it closely, BL means all mangas related to love stories between men. Yaoi is an acronym for « yama nashi, ochi nashi, imi nashi » which means « no climax, no ending, no sense » - if I remember well. So a yaoi can be a BL or not. It has nothing to do with the content of the story (explicit sex scenes or not). In the same way, "Shonen ai" translation is "Boy's Love". So Kaze to ki no uta is a BL, first one of the manga history. (▰˘◡˘▰)
I'd like to add that "yaoi" is more a kind of story telling than a genre itself, in Japan.
I think the heart of Thomas was the first bl in manga history.
Well still love for classical bl ⁄(⁄ ⁄·⁄ω⁄·⁄ ⁄)⁄
My mistake, after some researches, first BL manga is In the Sunroom by Keiko, from 1970. The Heart of Thomas was published in 1974.
Does this manga a yaoi? Or just bl