This song and "Veteran Of The Psychic Wars" were co-written by "Fantasy" author Michael Moorcock. Black Blade refers to the sword wielded by the normally very weak Prince of Melnibone', Elric, in Moorcock's books about Elric.
He was an albino with red eyes and very weak until he picked up the "Black Blade", Stormbringer. It was a weapon imbued with a VERY evil entity that took control and forced Elric to do tings he would not normally do, including killing friends, to feed the voracious appetite of the sword for SOULS! Stormbringer would literally suck out the souls of it's victims which sent them to the evil god Arioch.
Elric was even forced to kill is beloved. His life was strengthened greatly by the sword but the cost was very high. Usually Elric felt like a slave to Stormbringer, but he was addicted to the health and power the sword gave him and allowed him to be a much stronger ruler of Melnibone'.
When I first heard the song it was only a few months after reading Moorcock's books and I cheered at the fact that someone wrote about Elric and Stormbringer.