Update: here’s a deeper and better-informed explanation of blue notes.
Blue notes are a big part of what makes the blues sound like the blues. Most other American vernacular music uses blue notes too: jazz, funk, rock, country, gospel, folk and so on. In the video below, John Lee Hooker hits a blue note in just about every single guitar phrase.
For such a foundational element of America’s music, there’s a surprising amount of confusion as to what a blue note is exactly. So allow me to clear it up: a blue note is a microtonal pitch in between a note from the blues scale and a neighboring note from the major scale.