Comments here gave me a few ideas for uses for blackrock:
1) Change the color of the weapon to something dark, close to black.
2) Use it to enhance 1 property on an item, even if it pushes it above cap, by X%. High chance to fail, chance to break item, 1 time use.
3) Weapons made of blackrock have an item property cap of XXX (Won't include leather/cloth, for metal armor and weapons only). They are brittle, and quantity required will vary based on item being crafted.
4) Can make plate armor medible without using a property slot. Takes 25 blackrock per piece. (Come on.. Plate is useless.. Throw those of us who like to be stylin' a bone!)
Random thoughts.