Why do some people have black gums?

its kind of weird some people say thats its due to a skin pigment, but its not neccssarily all black people get it. I have seen some black people with pink gums and others with black ones. I have seen countries where the people have predominantely black gums but then you will see some with pink gums. It doesnt seem to be confined to a geographic area or is it. why is this?