What did the music industry call recordings from the 1920's until the late 1940's that were made by African Americans artists and produced mainly for sale to African American listeners, or made by "Hillbilly" artists and distributed mainly for sale to rural southern listeners?

They were called Race Records

Explanation:

Race Records were recordings of music made by African Americans, and directed primarily to African Americans. The genres were: blues, jazz, and gospel.

With time, White Americans began to listen to race records as well, influencing the music that they themselves produced. For example, many white people began to play jazz and blues.

d.

the northwest territory became the most densely populated area in the u.s.