Absolute blackness is a hard black material. It is found only on the Riotous Beaches, in shards, and is jealously defended by the native pebble-soldiers.
The accepted theory is that the shards of absolute blackness are actually the shattered remains of Blanksword, The Void Blade, sent back in time somehow. The shards were collected and forged into Blanksword, thereby creating a predestination paradox, but that's timesorc'ley for you.
Absolute blackness was assumed by many to be the physical manifestation of Voidnes, but when the timesorc'ley origin of the shards was promulgated everyone quickly lost interest in the whole thing. (Timesorc'ley has that effect on non-Chronosorcellors.)