I love Helen Merrill's recordings from the mid- to late-1950s; their aesthetic really captures the era for me.
Anyway, I've always wondered: what makes someone a jazz vocalist (as opposed to a pop vocalist)? In today's music the question is self-evident, but in the 1950s it seems more ambiguous. Did it have to do with how they marketed themselves, or the musicians that backed them (eg Clifford Brown in the case of Helen Merrill)? Obviously Judy Garland was not a jazz vocalist, but she sang a lot of jazz standards.
I suppose Nat King Cole is an even better example, because some of his recordings were "jazz" and others were "pop." But did his vocal style itself change?
Thank you in advance, DL, for helping my unsophisticated ear understand.