You say it can't be black cuz it's LIGHT....Well what about a blacklight
A blacklight is both black and its light. Technically a blacklight is just really deep purple and they have purple saber blades so it could be done. Surely there's a crystal that could do it. If not one could be created like the red lightsaber crystals. It wouldn't be perfectly black but it'd still look very imposing and cool in a lot of circumstances(except in pure daylight). But it could be done.