A "genre" is just a set of expectations you could reasonably apply to a large set of items. If all or most of your expectations when you hear a "blues" song are met, then obviously it was useful to call it "blues." If it had a few elements that remind you of that, then maybe "bluesy" would make sense. If there is almost nothing that anyone could expect based on hearing other music before then the piece in question HAS NO GENRE. Genre is not a quality absolutely inherent to everything. It only happens when a large enough quantity of something is able to be identified with a set of distinguishing characteristics. When a new genre starts, it isn't actually a genre until we've heard enough of it to know what to expect.
Categorizing things into styles and genres is a natural and useful part of human culture. Part of artistic, creative interest is in deviation from an existing norm. Those norms must be identified and embedded in the listener's unconscious in order for deviation to have any effect. On the other hand, some art is not intending to deviate, but is merely not related to any known genre, and people's openness to such new things is quite varying.
The future is promising. By using tags/labels in the digital computer world (in programs, like this blog site, that do not limit the number of tags versus the awful mp3 genre id tag that requires a single listing for any mp3), a piece with blues elements can be tagged as "blues" or as "bluesy", without sticking into any box. Tags are not exclusionary or restrictive. We may use as many or as few tags as are useful. Hopefully we will see the extinction of arguments that say, "this piece isn't rock, it's funk-blues." Instead the discussion can be "does this have enough rock elements to have a rock tag? I know it should be tagged with funk and blues, but maybe rock too?" In the past, a record had to be put in either the rock or the blues section at the record store, unless they had copies in both, which was very impractical. Now, an mp3 at an online store can be in both categories at once, and others as well. And we can choose to search for only songs that have both rock and blues tags if we like. Lots of people have tried to put things into one box or another, and those who rebelled tended to reject the entire concept of genres and labeling. I think tagging and digital technology liberates the discussion. I hope this will also encourage musicians to take a much more fluid view of their own stylistic identities.