“Three O’ Clock Blues” was the record that launched B. B. King toward stardom. He wasn’t the first to record it; Lowell Fulson had cut it in 1948 and enjoyed success with it. In 1951, King was a dee-jay on WDIA in Memphis known as The Beale Street Blues Boy, which was later shortened to Blues Boy and finally B. B. He sang on his show and did his own jingles, as well as spinning records. He had started recording in 1949 as well. His 1951 recording of “Three O’ Clock Blues” for RPM Records was actually done at the YMCA on Lauderdale Street in Memphis, using a portable tape recorder, then something relatively new. The owners of RPM, the Bihari Brothers, had fallen out with Sam Phillips who owned the studio they wanted to use (Phillips later owned Sun Records). So, they went to the Y, and what they did there made blues history.

Official B. B. King site