Sharp Uchigatana is finally at +5. From what I could tell:
4-1 black skeleton near the boss room has a 10% chance to drop Bladestone Chunks. Out of 40 tries I got 4 chunks. Providential Ring did not seem to make a difference one way or the other neither did being in Human form vs Soul form. I also got a Pure Bladestone from that one, out of 40 tries, possibly indicating a 2.5% chance but YMMV. Did this on pure black tendency, per recommendations from this thread. (thanks for warning me about the 4-1 1000x folded katana phantom and other nightmares that spawn on pure black, good job). He is the faster way to get Chunks, versus going through all of 4-1.
4-2 black skeleton behind the hidden wall drops Pure Bladestone. He has more health and deals more damage than his 4-1 counterpart. Not sure what the percentage is, but he did eventually drop it. I'd say go for Chunks first in 4-1, and if all you need is the Pure, then start farming 4-2.