@slim mentioned in this answer that:
In Blues music for example, some notes are deliberately flattened -- but others are not, and a listener with experience of the Blues would still spot badly pitched notes.
While I have long been aware of flattening and partial bends in blues, I had not realised this was a standard part of blues and can't seem to find any definitive description or instruction.
So my question is:
When will notes be flattened, and by how much, in the blues tradition?
Tim is correct that it's about the 3rd, 5th, and 7th, but I don't agree that in the blues they are flattened by exactly one semi-tone. That is an approximation when writing down the notes or when playing them on a piano, but on any instrument on which in-between notes can be played, these notes will be intonated differently. Especially the 3rd and the 7th will be played at a pitch that is somewhere in between the corresponding minor and major intervals. So when playing the blues scale, traditionally notated with a minor third and a minor seventh, these two notes are usually played a bit sharp.
In my perception things are a bit different with the b5, mainly because that note is neither part of the major scale nor of the minor scale, so it sounds a bit off anyway, even when played as a perfectly intonated b5. So I think that that note is usually not changed as much as are the (b)3 and (b)7. However, often notes are bent between the 4 and the 5, so in practice any pitch around the b5 will be reached, but not as a stable note that is held for a longer time.
I thought that the flattened notes in Blues were the 3rd, 5th and 7th. However, they're traditionally taken to the next semitone down, as in C, E to Eb, G to Gb and B to Bb. That puts them squarely on the notes mentioned, rather than 'just a bit flat', which may be what's mentioned here. The same three notes are sometimes hinted at, particularly on instruments which can 'play in the cracks', like guitars, by partial bends that are often called 'quarter tones'. Best ask @slim!
Every note in the blues scale gets bent all the time. Although we almost always bend up, we can increase the tension of a bend by approaching the target note, but not quite reaching it (which is hitting it flat.)
The third and seventh are only flat relative to and clash with major and dominant progressions. Only the flat five retains its tension in minor over the I-7 chord.
This is all a vast oversimplification because the blues scale is always played on the root of the key, not chord, so as the chords change so does the sound of the scale, and the notes you are more likely to bend also change. So be prepared to bend them all.
Be very wary of Wikipedia. Btw the 7 and 8 note blues scales on it are bogus and yes I read all the articles they cited.
It isn't really correct to say across the board blues notes are flattened.
In published, notated blues, like those of WC Handy, the so-called "flat third", may be notated as a raised ^2 scale degree over the V7 chord.
The other thing I understand is the "blue note" that can be added above a major triad is roughly a minor seventh above the root, but it is a little bit flat compared to equal temperament. Some say the "blue note" is the harmonic seventh, the seventh harmonic of the harmonic series. In notation those blue notes will normally be written as if minor sevenths above the chord root.
It also seems important to point out that the two examples above are not the same thing. The first is an altered fifth in a dominant chord. The second is a particular tuning of a minor seventh. You can refer to both generally as blues notes, but the second one is a specific interval with a specific name, the "blue note."
