A: Initially the blackstone in a weapon is supposed to increase your chances of hitting vulnerable areas (critical blows, not always the bloodied critical you will see, but any blow doing more then the normal weapon damage vs armor part)
Blackstones currently suffer from a bug. When placed in a weapon, blackstones cause the wielder of said weapon to be prone to take more 'critical' hits. The more powerful the ston the more noticable the effect. When placed in armor black stones will cause the user of that armor to suffer instant deadly or near deadly damage, resulting in a 1-hit knock out. its best to avoid using them until the bug is fixed.
A2: This one you will have to find out ig