Actually, it is quite easy to get. I had a luck of 7 and bure black WT in 4-2. At the beginning, there is a secret passage in one of the stairwells leading to the altar. Behind that stairwell is a dual Katana black skelleton (NOT a black phantom skelleton). I was farming from him and got a pure bladestone my first time. I kept going back to farm chunks of bladestone and found several more pures.
He's not that hard to kill. Out of the castle into the clearing, WALK with your shield raised. As he moves towards you, lock on and walk past him. He'll do his jump-slash type thing. Run up and stab him in the back. As he gets up, hit him twice. Pause for 1/2 sec, reset yourself behind him, then backstab him again. Repeat until dead.
If you do it right, you can actually backstab him back into the castle passage so you don't have to worry about those flying guys constantly shooting at you.