Check the schedule of the service(s) in advance. Many chuches have more than once services, and not all of them include choirs or other type of "black gospel music" in which you are interested. There can be a long line of visitors, too! Early morning services tend to be less crowded.
Some churches may have certain standard for attires (e.g. no flip-flops, jeans, tank tops, shorts, etc.). Better to dress conservatively.
No cell phone, video, or camera unless you are permitted by the church.
No behaviors that distracts worshippers (e.g. eating, talking, etc.)
Be prepaired to stay for the entire services, which could be longer than 2 hours. It is considered rude to leave in the middle of the service. If you only want musical performances, Sunday Gospel Brunch at the Harlem restaurants or B.B. King Club & Grills in Times Square are your choices.
Make a contribution to the donation basket. Churches are non-profits and your support will be appreciated.
Again, worship services are not musical performances.