Blues, jazz and soul

I've just received an email from Amazon alerting me to offers on "Blues CD's". What strikes me is how arbitrary that category can be. According to Amazon, it includes Dinah Washington and Etta James for example.

Clearly blues, jazz and soul all overlap with each other and are all rooted in black religious music. I think it's also probably true that many of the distinctions within African-American music have been made by white critics rather than the musicians themselves. Ray Charles for example is usually categorised as a soul singer but could just as easily be described as a blues, jazz, R&B or even country singer. 

I read an article in a blues magazine a few years back about the club scene and record buying trends in black areas of Chicago which pointed out that many of the people seen as blues artists within their own community are seen (and dismissed) by white blues critics and fans as soul acts. 

I know whose judgement I trust...