Short answers, pulled from the story.
Black-appeal stations were radio stations that programmed music specifically aimed at the African American audience, at a time when mainstream radio was losing its audience to television. They demonstrated that targeting a specific community with a defined sound could build a loyal listenership, a strategy that later became the foundation of the Top 40 format.
WDIA in Memphis claims to be the first black format radio station. It was programmed by Nat D. Williams with a rhythm and blues sound and also featured discussions of race issues by black announcers. Blues great B.B. King started his career as a disc jockey at WDIA.
In 1949 only four stations aired a black appeal format. By 1952 there were around 200 such stations, and by 1956 the number had reached 400. The format spread to cities including Birmingham, New Orleans, Nashville, Cincinnati, St. Louis, and Washington, D.C.
Black-appeal stations were a primary channel for the music that Cleveland disc jockey Alan Freed named Rock and Roll, a blend of rhythm and blues, gospel, and country. Through these stations the music reached jukeboxes and young audiences across the country, helping establish Rock and Roll as a mainstream phenomenon.
Radio was losing its core programming to television. Comedy, variety, and drama shows all migrated to the new medium, taking their audiences with them. Major sponsors were eager to follow, and ratings fell sharply, leaving stations scrambling to find a new way to attract listeners.
Black-appeal stations proved that aiming at a specific audience rather than trying to appeal to everyone could raise a station's ratings. That principle became the basis of the Top 40 format, in which stations programmed a defined set of songs for a niche audience. Many stations that adopted Top 40 rose from the bottom to the top of their local market ratings.