It is correct that if you have let us say a blues scale and a minor scale in f.ex A the minor scale will also cover the tones for the blues scale. But trust me, it is very useful to practise both. Your teqnuiqe and feeling develops from doing it. I have found myself doing lots of solos in a minor scale and then blended with blues scale. This often sounds very cool. What I try to say is that if you just practise the minor scale your mind will not focus on anything else to blend your style with. It broadens your own personal teqnuiqe in my opinion to have a good control over the blues scale as well.
Good luck!