I have been doing this for a looong time. Pure black tendency, 51 luck stat and with the large sword of searching but still i havent gotten lucky yet.
Is this really the best way of doing this?
My theory is that luck stat has no effect what so ever and the pure bladestone will be completely random drop and you will have the same chance of getting it regardless of your luck stat...
I tried different weapons but the dragon weapon seemed to get me closest by giving me chunks but im not sure if its a factor which weapon you use...
So after 4 hours, not even joking, i got 167 shards, 17 chunks and 6 utchigatanas.
Even though i love the game, i hate the drop rate of this particular stone:)