It seems like there are kind of two different questions inside of this thread.
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Positions
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The Blues Scale
Short answer on positions: what hole is your home base?
1st pos: 1,4,7,10
2nd pos: -2,3,6,9
3rd pos: -1 -4 -8
4th pos: -3", -6, -10
For your first 5 or 10 years, probably 1st and 2nd position is basically all you need.
Maybe some 3rd position.
Regarding the Blues Scale, it’s just a group of notes that you can draw from to improvise over the blues that sound good. You can use other notes too, but it’s a great jumping off place, and 2nd position is the most common place for Blues.
@kjlarochelle made a cool graphic of all the blues scale notes that are available on a standard harmonica without overblows or overdraws:
Missing the -3’ in the upper octave is the note that harmonica players always wish they had, which is the nice thing about a Parrott tuning.
Hope that helps! Rock on!