As a major Lore head, I can say that it's not the Black crystals that bug us. Black crystals are totally acceptable. It's when people ask for the Darksaber, the unique, ONE-OF-IT'S-KIND legendary lightsaber, that gets my Nerd Rage running. The Mandolorians stole it from the temple and have kept it hidden until the Clone Wars. To suddenly see it on the Cartel Market or seeing thousands on the fleet is not just lore breaking, it's very tacky and trivializes the value of the Darksaber. Not to mention that if the Mandos were discovered to possess the most powerful lightsaber in the galaxy this whole time and not hand it over to the Sith, Mandolore would have burned to cinders and every Bounty Hunter would be killed on sight.
Black crystals: Cool. People have been trying to recreate the Darksaber's power for centuries. It makes sense to see them.
Darksaber: Not cool. You should not have one. Breaks lore into billions of pieces and I will not play a game were Lore is treated as such.
Yes, I would love a pure black crystal myself, but have seen some people using the darksaber as a reason not to do it, which is why I said ultra-rare, but if people want it as a regular directly buyable crystal, great, do it.