How old are they? Have you heard any crowing? Have any of them started to lay eggs?I bought 6 black majesty maran chicks. They were supposed to all be hens. One has begun developing differently. Comb is getting bigger, tail feathers stick up and out and it has a white spot on the sides of its head just below the back of the eye.
The white spots are earlobes. Some chickens have red earlobes, some have white earlobes. If you look at photos of Black Minorcas, they have white earlobes like that too. (Not saying you have a Minorca, just mentioning them because they have the same contrast of white earlobe on black chicken: it is quite eye-catching!)
When they are all the same breed, a cockerel (young male) will usually grow faster and bigger than the pullets (young females). He will usually develop a big red comb and wattles when the pullets still have tiny pale combs and watlles.My hens are huge and fluffy. This one is rather thin by comparison. Could it be a rooster? It has developed much slower than the others.
If this bird is smaller, thinner, and slower to develop, then it may be a different breed. Or since Black Majesty Marans are a mix of some sort, this one might be a slightly different mix than the others. It can be harder to tell male vs. female when they have other differences too.