This is pretty much the only use it still has. You can basically create a paragon anywhere, and black rock infected monsters have both a chance to drop a box (if they drop a treasure map) that will be one level above their map drop (similar to paragon chests) and (I believe) an Ilshenar minor artifact. Which means when you are hunting (whatever you hunt) to get your anti-virtue artifacts you can blackrock infect those creatures to have a chance at an anti-virute artifact, an Ilshenar minor artifact (I believe), and a box with a level 6 treasure chest and some other loot.its fun to infect creatures....sorta
I actually prefer carrying the blackrock vs casting the spell, as then you can control if you are going to infect anything. Let me tell you it makes for an interesting time when going to do a champ spawn and you have cast the spell or someone is carrying blackrock.Also You just need to carry a piece of a rock with you to infected monsters. Casting the spell helps but if you have a fighter you can just carry a small peice and monsters will start to spawn.