[quote name='Bilbo' timestamp='1339359172' post='1687459']
I was saying that you may be fooled intpo thinking a Bb blues is in Bb because its called a Bb blues but it isn't and that, when the mode changes from Bb mix to Eb mix, it is a key change, but the chart will remain in the same key that it started in and not change key. My point is that, if you know what key the tune is in, don't be fooled into thinking that the whole thing remains in that key for the whole tune..
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A Bb blues is in Bb harmonically. I don't think we hear a 12 bar blues as constantly changing key - it's really a variation of an 'old as the hills' I, IV, V surely?
It [i]could[/i] be described differently, but then to be consistent we couldn't really admit chord IV7.
If I were to read a blues in Bb, I'd expect the key signature to have two flats and the usual accidentals that signify 'blues in Bb', and, happily, the majority of music I've encountered does this.