In Ultima 6, the avatar found a black moonstone at the very beginning. It has reported that this was fashioned out of blackrock and it allowed him to cast the gate spell to all manner of locations. You could effectively hop all over the world as quick as you wanted.
Not only that, but the static moongates exist in areas where blackrock is buried in the ground.
UO did have moonstones for a while, which allowed travel between facets.
Blackmoor also has blackrock in it.
According to U7, blackrock is nigh indestructable by
any means, magical or otherwise, with the exception of magic used in combination with magnetic fields (which should make the stuff sculptable). In practise, it made it explode. Furthermore, blackrock was being mined in the game with standard machinery, so presumably it's not as solid as all that...
Blackrock is to magic as lead and concrete is to radio waves.
The "black gate" is a moongate entirely formed of black rock. It's powerful enough to hop between worlds. A few of these have popped up in both the original Ultima games as well as in UO.
Can you still infect mobs with this crap?
As of the appearance of the Crimson Dragons, you cannot. There have been quite a few requests to have the system re-activated.