[FrankHudson]

I suspect the "between notes" aspect is based on those folk scales which may not be tempered like keyboard instruments, use microtones which divide up the octave into more steps, or may be based on degrees of "out of tuneness" sourness used for effect.

B. B. King, a master of finger vibrato said he learned to do it to copy his relative Bukka White's bottleneck slide-guitar playing. Slide playing in blues is another way to get those microtones.

Of course vibrato wasn't invented by American blues players (though most of us guitarists sure owe them a debt). String players, many who play fretless instruments have been using it for years. And besides voice, other folk instruments around the world allow it (sitars with raised frets for example).