Rakata vendor on Republic space station sells items with black/blue crystal. You can get tokens for them from Hardmode and nightmare operations.
But you don't want black-blue crystal. Black sabers are one of things that distinguish lame players with no respect to Star Wars from everyone else.
Besides that - even ascetic wise they are no more than glowing sticks. It's like running with training saber while everyone around have real lightsabers.
Thanks for telling us where these were located. I for one think that these colored light sabers look cool and give you something to stand out because of your achievements.