So if there are two DPS in the dungeon and one has BLM leveled to 49 while the other has BLM leveled to 50 then only the 2nd one should be able to "greed" BLM-specific level 50 gear?

Sorry, that just doesn't make sense to me. "Greed" means that you can't use on that particular class, but you want it. I don't see why the 2nd DPS's "greed" should get precedence over the first one's...