Blues Harp Scale
There is much discussion over what exactly is a blues scale. I've listed a basic scale that is used in blues. These are the notes that sound good for blues. Practice this fast and slow. Almost all blues riffs use a piece of this scale. The three hole draw bent is a parial bend. It should drop the three down the half step needed to make the minor. This note is the heart of the blues and can vary from unbent (for a major country and western music sound) to more than a half step for a dark minor sounding blues. Your ear should tell you when you have this bend right.
There are a couple of notes below the 2 draw that are repeats of notes in the middle scale.
There are some blues notes above the 6 hole blow, but I not fond of the sound of them. You can use the high end of the harp, but the low end sounds so much better. The top 4 holes in a harmonica should be used in first position and then only occasionally.
In an interview with Kim Wilson, Carlos Colina asked Kim what was his greatest strength in playing the harp. Kim answered "I have a good 3 draw bend".
The exception will be the riffs where the little menu comes up and it allows you to play in first or third position or correct the key. You still will use a "C harp, but they song key will be "C" or "D" accordingly.