I'm pretty sure it's the key of G Major, in the blues scale of G Major the 3rd -5th & 7th notes of the scale are flattened. Since the 7th note of the G Major scale is F#, you'd flatten the 7th note to F. No other notes are sharp in the key, and the notes for the F Major chord are F-A-C, so the F Major chord would diffently work in the key of G. Great tune, hope this was some help to ya.
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