Any guitar you play the blues on is a blues guitar. It don't need to be ladder braced but it does need to be loud and have a good thump on the low end for rolling the bass. Not overly bright and with a good full midrange. If there is a wood associated with the guitars played in trhe 1920s and 1930s if anything it would probably be birch.
I do agree with a previous post about round soundhole archtops. There is just something about these guitars. If you an find a good one, it will combine the punch and midrange of an archtop with the fatness of a flattop. My favorites have a slight muted warmth to them.
Here is a nice pair what might be considered "iconic" blues guitars - an early 1930s National Duolian and mid 1930s round soundhole archtop Kay Kraft with birch back and sides and the Zorzi adjustable neck. Then again, I also play a 1960 Gibson J-200 and a 1956 Gibson SJ which are both great blues guitars because, yup, I play the blues with them. If I played folk music I would probably be telling you they are great folk guitars.
