That is an old map that was primarily guesswork based on the information available at the time. Black Dragon Island is not on it because the location wasn't known. BDI is located off the Southeast coast of the Northern continent.
If you believe this map, although I'm not completely sure of its origins, it shows the location of BD island to be off the coast of, and directly south west of, the Republic of Tinto. I'm pretty sure this map is canon although I can't remember why, but I think it was included in an artbook for one of the games. Anyway, if anyone else can verify it, or knows of the maps origins then let me know! Cheers.