As long as I'm blathering, one more thing to keep in mind about the difference between classic yuri and BL is the different depiction of love. In yuri, love most often brings sadness or pain. It's either an "impossibility" or "impermanent".
To illustrate what I mean, I'm thinking about a one shot by Kihara Toshie about a young woman who comes to meet someone at a hotel, as they promised while in high school. As it turns out, this person was a woman, and they shared a
class s type of relationship in their youth. The women she was supposed to meet does show up...with a fiance in tow. This paints the actual lesbian as a "tragic figure", one that was unable to move on normally like everyone else and is doomed to a life of loneliness as she literally walks off into a foggy morning. In "Shiroi Heya no Futari", the protagonist, after her beloved is murdered, decides to live the rest of her life in quiet, lonely despair.
BL, on the other hand. While you have elaborate tragedies ala "Kaze to Ki no Uta", two other prominent BL stories that shaped the genre are
wildly different. In "Touma no Shinzou", Juli eventually comes to terms with the fact that he loved Tomas, and decides to use it as a source of personal strength for the rest of his life--even if Tomas is no longer living. In "Mari and Shingo", even after Shingo admits that he'll never be able to love Mari romantically, Mari will always be the most important person to him. The strength of that friendship might sometimes cause pain, but it's ultimately a positive thing.