I've not personally seen a Black Ostrich (well, a real one that is...I have seen at least one fake) anywhere myself, but I do know there were several colors of the Rive Gauche in ostrich, so it's certainly possible. I think that if it did exist, it would be a pretty hard bag to track down, though.
Hopefully someone else will know for sure whether this was ever a leather/color combination produced by YSL.