Synonyms also work with regards to creating memorable names for your characters, something that stands out.
Dark is a commonly used word in comic books, synonyms that could be used in place of 'Dark':
Midnight
Umbral
Dusk
Murky
Gloom
Tenebrous
Tenebrious
Shadow
Stygian
Veiled
Sable
Ebony
Grim
Bleak
You can open up options by considering what the name is describing; is it appearance, a description of their powers,, or a frame of mind or any number of other reasons.
For instance, back on Legacy I had a Ice/Dark character and 'Black Ice' was pretty obvious (and taken) so I selected 'Sable Drift' instead and found it preferable.