If "blues singer" means "someone who sings blues," then most recording artists were, up till relatively recently, blues singers. Much, maybe most, popular music of the 20th Century was at least blues-influenced when it wasn't outright blues, which means that a whole lot of rock 'n' roll, rockabilly, classic pop, country, jazz, swing, gospel, and more is in fact the work of blues singers.
My point -- an obvious one (I should hope) to any informed reader -- is that where pre-war black folk musicians were concerned, record company a&r men (and they were nearly all men) insisted on blues because releases labeled "blues" (issued in the so-called race series) had proved popular and profitable. But we know from field collecting up through the middle of the century that so-called blues singers had other kinds of material in their repertoires. The fact that only the blues part of their song lists got recorded commercially gives us a misleading idea of what their tastes were and what the totality of their repertoire was.
It also explains why all kinds of songs that weren't blues at all suddenly got "blues" attached to their titles. It was because the labels thought the b-word would enhance sales.
At times, non-blues material got recorded commercially, almost by accident, by "blues singers." I have no desire to carry on at length, and to no point, on that subject. I refer interested readers instead to the late Paul Oliver's superb Songsters & Saints (1984) for specifics.