Have been killing the Blade of Crota, Dog knows how many times since it it was added to the maps (one or two per area patrolled), so couldn't give you an exact number. But a friend and I went farming for it today on Fist of Crota mission. He had it after 7 attempts, took me 15. Then about 20 minutes after that (patrol missions around the Hellmouth) took out another "Blade" with my Husk and now I have two of them!