The blues singers were really the first jazz musicians and they set the stage for improvising with instruments. Ma Rainey and Bessie Smith hugely impacted the playing of Louis Armstrong (one of the most original vocalists in jazz beside being the original Trumpet God). and King Oliver. These men, in turn, went on to create a whole new instrumental language. Jazz vocais, although remaining a vital jazz form, also developed into a popular art form but was not considered genuine jazz much of the time. Frank Sinatra and Sammy Davis Jr., though truly jazz singers, illustrate this point.
Nat King Cole
Lambert Hendricks & Ross