Where the mithril crossbow is the standard issue of the ranged Black Knight, the black sword is standard for footsoldiers.
'Black', you may be aware, is not a material. It is instead a descriptor for steel that has gone through a process similar to vulcanisation. There was a time when the vulcanisation was performed by the dragon Xikotal, as a gift to the isolationist knights.
With his death, the process moved towards the demonic. The resulting quality is lower, which has made the original, dragonfire 'black' metal sought after in Dowdun Reach.