Blüdhaven is the city that Nightwing protects as well as Gotham City. Located not too far away from Gotham, Blüdhaven's crime rate is almost at Gotham's level. Nightwing has encountered many criminals there and even some of Batman's enemies.
Blüdhaven is never seen in the series but has been mentioned numerous times by thugs throughout the series until the last game. Nightwing talks about it in Batman: Arkham Knight. In Batman: Arkham Knight the Harley Quinn DLC is set in the Blüdhaven Police Department, and the GCPD Lockdown DLC has a part set at a warehouse near the Blüdhaven docks. It was also given a mention in the A Flip of a Coin DLC, revealing that Two-Face's gang proceeded to rob various banks in that city as a fall-back plan in case their attempt at robbing banks at Gotham failed, with their succeeding largely because of Nightwing being out of town due to his trying to stop Penguin's weapons laundering business via North Refrigeration. It also played a fairly major role in the game itself, as it was the place where Penguin was laundering weapons to via North Refrigeration.
Blüdhaven was a former whaling town, which was officially incorporated as a "Commonwealth" in 1912. The town had a generally poor socio-economic populace, owing in part to failed efforts to transform itself into a manufacturing and shipping center.