well you can find darklight in the latest set of hard mode dungeons. Hard mode brayflox and halatali, as well as in city of amdapore. The first binding coil of bahamut and the lyberynth of the ancients you can run to your hearts content as well as the binding coil being given an echo buff, so it should be very farmable. Leviathan was added, as well as king moggle mog extreme, fight before that has echo buff, but in theory you'll most likely only see the echo buff pop up on titan/titan extreme. I just woke up, but that should give the gist. Also you can dye darklight, there's the glamor systems, so vanity is in.

EDIT: Also you can further upgrade your relic weapon if you you're the sort that likes grinding, but the relic upgrade is entirely optional since there are other weapons that are easier to obtain that are just as good.