Mcmoo, I agree that we shouldn't be bound by the categories, and that it's not really necessary to even pay much attention to them. I play the music I want to play, whether or not it fits neatly into a defined category, and I would encourage others to do the same. However, if someone asks "what is blues?" I assume they want a definition of a category. If other people feel there's no value in this sort of analysis, they needn't participate.
As for a lot of the rest of the comments here, they seem more intent on defining blues as any music that is heartfelt. I think that does an injustice to other forms of music which might be just as emotional and sensitively performed, but do not meet the historical, technical or structural definition of the blues. In my world, heartfelt and sensitively played music is "good music," regardless of what genre it falls into. There's good blues, folk, jazz, rock'n'roll, classical, etc., and there are also plenty of examples of bad music in all of those categories. What constitutes "good" or "bad" music within any category is subjective, of course. But I don't think the "good music is blues, bad music is not" angle really makes a lot of sense.
Submitted for your consideration, with all appropriate humility.