Do you live in a city that has a gay bar or club popular with black men? That's how I met some of my black lovers (including two boyfriends and some who could have gone on to become boyfriends if I had chosen).
Be clear however why you want a black man (or any other man) as a boyfriend. In my experience there are many white guys who objectify black men. I'm not saying that this is the case with you, but any sensible black man will realise if you're not interested in him as a person, which includes his vulnerabilities, his hopes and aspirations, his opinions and beliefs, his values, etc etc