When I was in elementary school, in the 1950's, I read an article about blues, in a special advanced reading class. Of course, at that time I had no idea what "the blues" meant. But I remember it said that if a European band heard there was an American in the club they would usually play the blues. And for guests from other countries they would play that country's national anthem. So some people got the idea that the blues was the US national anthem.