Blackmarrow is a namedSkeleton Engineer enemy who was planned to be located on Crux Prime. However, before the release of the world, Blackmarrow and another named enemy, Hardhead, were removed. Blackmarrow is still present in the game's coding, though, and can be spawned by Mythrans.
Blackmarrow was spawned by a Mythran in Nimbus Plaza on January 3, 2012.
The name is based on bone marrow; a material found in bones.
Blackmarrow, unlike the other named Skullkin (excluding Hardhead), is not present in the Ninjago storyline.