When you say 'black,' do you mean a black Wildtype or do you mean a black Melanoid? Either type can be black. Are you more upset because he is grey or because he is Wildtype?
When I bought my first axolotl he was labelled as 'black,' and after a few months he turned a chocolate brown colour, with a speckled grey tummy and the Wildtype eyes. I realised that he had been quite young and perhaps hadn't had the right foods to help him grow. I realised he was a Wild Type when I spotted his eyes, and knew then that the LFS perhaps didn't have enough information about the axolotls to know what colour they were considered within the hobby. Often people either breed independently and thus don't necessarily use the labels we use - case in point, my 'albino' axolotl who is actually a Golden - and so it can happen that what we are sold is not necessarily what the axolotl will become, or is. Axolotls also change vastly in their first year or so, so what begins as a black axolotl can quickly change to a chocolate brown axolotl, like my Tooth.
I understand that you are upset, but I think I'd like to know what the personality issues are with this axolotl. Is he/she aggressive or snappy?
If you are after a specific type you must remember that people can't always promise that what the axolotl is now is what it will always be. They change vastly. It's also worth remembering as well that you can tell a WT from a Melanoid by the eyes, so you'll know what type you are getting.