The Black Mask is a high-end costume shop in Absalom's Ascendant Court, carrying masks, cloaks, and everything from common disguise kits to costumes used by parade or theater performers. Behind this facade, the Black Mask is a temple to the god Norgorber, offering worship once the shop's doors close at the end of the business day. Assassins and rogues of all descriptions find that these clandestine meetings and the masked identities of many patrons makes the Black Mask a superb meeting spot for all manner of shady deals. Locals are frightened when the Black Mask starts selling skinsaws (leather costumes with ghoulish faces) because these "sales" always seem to precede strings of gruesome murders, though these events have never been officially connected.1
For additional as-yet unincorporated sources about this subject, see the Meta page.