I'm dominican and that makes me black. So why do people call me mixed?

My father was born in Dominican Republic and my mother was born in France. They are both black meaning that their families were brought from africa to these regions in the world. Now if I'm not mistaken slavery was not just in america. So y is it when i say i'm black im instantly shut down...."no your not your dominican". Dominican Republic is a place not a race. no black american says im not black im american. so whats the deal?