To toss in the opinions of one who used to make tombstones:

Most people can't tell the difference between marble and granite. Both come in a wide variety of colors: here are some black granites and here's American Black from Rock of Ages. We used to use Swedish Black until the boss became disgusted, he said they'd started quarrying with dynamite instead of black powder and the resulting microfractures ruined the stone.

I didn't see black stone buildings in Kilkenny, but then again I wasn't looking. Could well be, could very well be.

Marble is just another form of limestone.

I always took the line to mean that there were black gravestones in Kilkenny. Having made some, I had no problem with the concept.