It is said that the original 1962 recording of "Watermelon Man" by Herbie Hancock is in the key of F. It makes a lot of sense given it's a bluesy piece, so we have a typical I-IV-V based chord progression.
However, here is where it gets confusing to me: the first chord is F7, but there is no minor seventh in the F major scale. Why is it then that people still say it's in F? Wouldn't it make more sense to call it F mixolydian? Or is it implied?
There isn't a 'rule' about what notes fit into what key - not really even an unwritten one!
Especially when considering jazz and blues.
Yes, I suppose 'F Mixolydian' might be a more accurate way to describe the key here, but all the transcriptions I've played from use the key sig. of 1♭, (Fmajor) rather than 2♭s (B♭/F Mix.).
It's what blues and jazz do: use a basic key, and use flat 3s, flat 5s and flat 7s. It's become the norm over many decades, and doesn't present a problem to us - not even pedants such as myself!
Listen to the recording (on the Herbie Hancock album "Takin' Off") with the sheet music and try to see if it makes aural sense to you. If you haven't listened to much blues-oriented music it might not make any sense at all! But listening to "Watermelon Man" is a good start. Then, try to understand why it sounds good! Remember that music theory is only observations and explanations about what our ears think sounds good.
Blues is a style that's non-diatonic (it does not conform to diatonicity rules whereby you stick to 7 notes in a scale). There a few guidelines that apply to blues (except when they don't)
In major blues, all chords are dominant (ie: major chords with a minor 7th).
Note that further on in the progression you find a C7, which has an E, which conforms to Fmajor (vs the Eb that F7 would have).
Major blues compositions are both minor and major at the same time (ie: you can play both the minor 7th and the major 7th).
The blues scale uses the flatted 5th.
It's these chromatisms that give a tune the 'bluesy' sound.
