Can black Amish people marry white Amish people?

Yes.

Amish accept converts from all races. It's very uncommon to have a dark-skinned Amish, because not a lot of converts are dark-skinned.

However in some communities there may be a little discrimination, particularly if the mixed-race couple decide to have a child. But, generally, most communities are accepting of this.

You should bear in mind that excluding someone because of their race or skin colour is a violation of your Human Rights. So if you are rejected by an Amish community because of skin colour, wave a copy of the Human Rights Act in their face and threaten to sue. Just because the Amish choose to live in the "past", it does not make them above the law.

The Amish-Mennonites and the Conservative Mennonites are said to be the most open groups of Amish and the communities with the most mixes of ethical races.

Your best bet is to visit the Amish and ask the elders.