Via a list of questions and answers about Batman: Arkham City, we now know that Black Mask will be in the game in some form via the exclusive Robin DLC. Here are the details.
The two exclusive Robin maps are “Black Mask Hideout” and “Freight Train Escape”. In response to the question of “what does Black Mask’s Hideout mean?”, Batman: Arkham City community manager SarahMay answered, “Black Mask has a presence in Arkham City”.
In addition to that, SarahMay confirmed that details for the Robin DLC for Europe and Canada will be confirmed later, and that Robin will have “his own very unique set of gadgets and moves”.