This is an odd mixture of the element of Earth and the mysterious Blackrock.
Blackmoor was to be a Necromancy Reagent. It was dropped by Liches as loot for a while (and later by Ancient Liches in the Dungeon Khaldun), and is now coincided Rare as it no longer spawns. It was part of the old cancelled necromancy system, and is now one of several sets of what are called Pagan Reagents. It can be stacked and weighes one stone each.
Like many things in UO, Blackmoor was first seen in the Ultima single-player games, specifically as a Necromancy reagent in Ultima VIII: Pagan.
Blackmoor • Blood Spawn • Bone • Brimstone • Daemon Bone • Dead Wood • Dragon's Blood • Executioner's Cap • Eye of Newt • Fertile Dirt • Obsidian • Pumice • Serpent's Scale • Vial of Blood • Volcanic Ash • Wyrm's Heart