Where the blue note comes from, the scales and 7th chords that carry it, the 12-bar form everyone jams on, and four ways to solo over it.
The blues grew up in African American communities of the Deep South around the end of the 1800s — out of work songs, field hollers, spirituals and call-and-response singing, with roots reaching back to West African musical traditions. Its voice was expressive and vocal: notes bent and slid between the fixed pitches of European instruments, landing "in the cracks." Those bent pitches became the blue notes.
In the 1910s W. C. Handy helped publish the blues to a wider audience; the Mississippi Delta gave us its rawest early form, and when players moved north the electric Chicago blues was born. From there the blues became the taproot of nearly everything that followed — jazz, R&B, rock & roll and soul all grew from it.
A blue note is a note sung or bent a little lower than the major scale expects — most often the ♭3, and also the ♭5 and ♭7. On a piano or fret you can't bend, so you play the flattened note instead. Two scales package the blue notes for you:
The minor blues scale is the minor pentatonic with the ♭5 slipped in (its blue note). The major blues scale is the major pentatonic with the ♭3 slipped in (its blue note). Same trick — take a friendly five-note scale and add the one note that gives it grit. (The blue note is shown in blue.)
Blues harmony runs on dominant 7th chords — a major triad plus a ♭7 on top (C7 = C–E–G–B♭). The three chords of a blues are all dominant 7ths: the I7, the IV7 and the V7.
Here's the beautiful trick. The chords are major, but the melody leans on the minor ♭3 blue note. Over the I7 (which contains a major 3rd), a singer bends toward the ♭3 — and that note isn't random: it's the ♭7 of the IV chord. In C, the ♭3 is E♭, and E♭ is exactly the 7th of F7 (the IV). The blue note is borrowed from the IV7 and rubbed against the I. That major-vs-minor tension is the blues.
Follow the blue note: the blue key in the IV7 above is the ♭7 of that chord — and the same pitch is the ♭3 (blue note) back in the home key.
Almost every blues you'll ever jam on is twelve bars long and repeats. It's built from just the I, IV and V chords in a fixed pattern — three four-bar lines, written here in slash notation. The last bar is the turnaround that spins you back to the top.
Common variations: a "quick change" to IV in bar 2, and swapping bar 12 between I and V. Change the key up top and the chart rewrites itself.
Swap the mood from bittersweet to properly dark and you get the minor blues. Same twelve-bar frame, but now the i and iv chords are minor 7ths, and the turnaround borrows a ♭VI7 falling to a dominant V7 — the aching "Thrill Is Gone" sound. The V stays dominant so it still pulls home. It uses the same chord / Roman toggle above.
Solo with the minor blues scale on the tonic — it fits the whole progression.
Because the whole progression lives in one key, there are several honest ways to improvise — from dead simple to properly sophisticated. Most players grow through them in roughly this order.
Take the minor blues scale on the key's root and use it over all three chords for the whole 12 bars. Because the chords are all dominant 7ths in one key, that single scale "works" everywhere — the friction of its ♭3 and ♭5 against the major chords is the point. This is the classic blues-rock sound and the fastest way to start playing.
Best for: getting going, rock and blues-rock, big expressive bends.
Forget scales for a moment and target the notes of whatever chord is sounding right now — the root, 3rd, 5th and ♭7 of C7, then F7, then G7 (arpeggios). Landing on a chord tone on the strong beats sounds "inside" and horn-like, and it makes the changes obvious to the listener. This is how jazz and vocal-style blues players outline the harmony.
Best for: a melodic, "singing the changes" sound; slower blues.
Treat each dominant 7th as its own tiny key. Play a Mixolydian or blues scale built on each chord's root as the chords move — C over C7, F over F7, G over G7. It's more work and more to track, but every note now fits its chord, giving a smooth, "each chord gets its own colour" sound.
Best for: jazz blues, making each change speak.
The pro compromise: stay mostly in one blues scale for its unmistakable feel, but bend toward the current chord — add the major 3rd when the I chord wants it, hit the IV chord's ♭7 (the blue note) when it comes round, and resolve to chord tones at the ends of phrases. You keep the identity of one scale while honouring the harmony. This is how most great blues solos actually work.
Best for: everything — the sound most seasoned players settle on.
Tip: the blue note is the flavour; the dominant 7th chords are the frame. Learn both and you can play the blues in any key.