"Real" blues has a very different connotation than "real" Folk. I think the essence of Folk lies in the narrative and traditional aspects of the form. Blues is different. The 12 bar standard structure is a way of identifying the form, as is the triplet rhyming pattern. Instrumentation and tradition are important, but not definitive. The essential aspect that does or doesn't make a song "real" Blues is primarily depth of feeling. That is the main reason that Clapton only occasionally qualifies, and Janis Joplin always did. That "feeling" is an interesting blend of ecstacy and desperation. It isn't a racial thing- Robert Cray only occasionally engenders real Blues, while John Lee Hooker always does. It can be electric,acoustic,or accapella. But Real Blues attains an intimacy between player and listener that no other music can.You certainly do Know it when you Hear it.