If you trust the seller then you can believe him but sometimes people can just be passing along a fish that was sold to them as something that might indeed turn out to not be what it was purported to be. Did the seller breed this fish himself? If so then you should ask to see the parents. The parents should be an electric blue and a normal looking JD.
Again, if you read up on these fish you will learn that you are not going to be able to determine whether it is a blue gene by eyeballing the fish. Just like you cannot look at a person with brown eyes and tell whether or not they can have a kid with blue eyes. The only way you are going to be able to determine if it is a blue gene dempsey is to either breed it to another blue gene or an electric blue jd and see if you get some ebjd fry. Other than that you can take it to a lab and have it genetically tested.
Just a question, do you know the difference between an electric blue and a blue gene? I just went back and read your OP and wonder if maybe you might think that they are the same thing?
It's not a bad looking fish so as long as you don't pay a fortune for it and don't have your hopes too high you should do alright. Good Luck.