Short answers, pulled from the story.
Jerry Wexler of Billboard magazine coined the term rhythm and blues in 1948 to describe recordings marketed predominantly to African Americans. Before this label existed, record companies used phrases like race music or sepia series to categorize black music for white audiences.
Billboard officially replaced the Harlem Hit Parade with its new Hot Rhythm & Blues Singles listing by June 1949. This category remained unchanged until August 1969 when it became Best Selling Soul Singles.
New Orleans musicians began incorporating Afro-Cuban rhythmic motifs into their work during the late 1940s at precisely the moment R&B was first forming. Professor Longhair listened to Perez Prado's mambo records and fell under the spell of Cuban influences while developing his rumba-boogie style.
Cleveland Ohio DJ Alan Freed started a late-night radio show called The Moondog Rock Roll House Party on WJW 850 AM in July 1951. Freed began referring to the rhythm and blues music he played as rock and roll after receiving sponsorship from Fred Mintz whose record store had primarily African-American clientele.
The Grammys added the Rhythm and Blues category in 1969 giving academic recognition to the field. By the 1970s the term rhythm and blues had changed once again and was used as a blanket term for soul funk and disco.