At my local Neiman Marcus they were referring to the cornflower bags as "denim." Perhaps the bag you were called about is the cornflower? Sometimes the SAs get the colors mixed up...
Also, since the bag is a return, you should check it very, very carefully. My store had someone return a fake "cornflower" (they bought a real one and returned the fake using the Neimans receipt). The SA who accepted the return wrapped up the bag and put it in the back with all the others. When I asked if they had any cornflower city bags in stock they pulled that bag to show me. I mentioned that the color looked different and that the bag had silver hardware. My comments prompted them to look closely at the bag and when I returned later in the day, the SA told me that the bag was fake. This happened just a couple of weeks ago, so really examine the bag closely, especially if the store normally doesn't carry Balenciaga, since the SAs may not know what to look for when taking returns.
By the way, Neimans assured me that accepting the fake was a fluke that was the result of a fill-in SA who didn't know policy surrounding the return of B-bags (must get manager to sign off, etc). I'm sure they will watch even more closely from now on!