To be honest, just stick with ShadowMarket. Who the fuck cares if the word 'shadow' has a negative connotation? Don't censor yourself. You should be proud of the name ShadowCash / Market. And especially don't Darkcoin / Dash this awesome project by rebranding. If whoever has a problem with this project's name, tell them to stop being whiny cunts and to go fuck themselves.
Its not a re-branding at all, Shadowcash will still be Shadowcash. Its just going to be a name for the market that will be running on the Shadowcash platform.