One of the main factors that makes The Blues sound 'blue' is the use of 'minor' 3rds and 7ths against major chords. The blues can have a tonal quality that isn't 'normal' major or minor. Some charts will spell out the chords for the blues as all dominants: e.g. major chords with the 7ths flatted.
A single substitution of a flat 3rd or 7th is enough to establish a 'bluesy' quality. Or even bending a note in that direction. Bix certainly loved to bend notes...something a lot of imitator don't seem to have picked up at all.