Is he barking out of excitement? If not, then no, it's not acceptable or normal. I got this four month old pup from the pound about 23 years ago. I didn't know at the time that there were black GSDs. Since I found that out many years ago, I believe he was a shepherd. He had the looks of a shepherd without a doubt. He was just black. I got him because I wanted a dog to ward off some people who weren't very nice. He did his job well. However, I have to admit that black GSDs scare me a little. Maybe the people that are afraid of him feel the same way. I don't feel that way about pups, but maybe they do. He may be feeding off their fear... so he's reacting to it. I think Grim would have a good ole time with that... seeing how far he could push it.
I don't know what your pup's personality is. Would he bark at people who are afraid of him just to yank their chain? I think you need to find a place to take him where you know people aren't going to be afraid of him. If he is yanking their chain, you need to stop that. Don't worry about people not knowing he's a GSD. I've had a few ask me about Grim, even. Some people think that the saddle back black and tan shepherds are the only 'variety' there is. Good time to educate!