Finally, over a year since I finished the game, I got the Pure Bladestone from the black phantom in Demon's Souls. I must have made that run hundreds of times (I went from level 80-ish to 198 in the process) and didn't get a bean (PB has a 1/200 chance of a drop).
Then he dropped 2 in a row.
So if anyone is looking for a pure bladestone (and is a ridiculous soul level), let me know.
The big problem now is that it opens up the Platinum as an attainable target. Very daunting.