Maceo Parker's blue notes in a James Brown classic

Why bend an F halfway down to an E?

I got interested in tuning theory because of the blues. The first instrument I learned to play well was the harmonica, and an essential part of blues harmonica is bending notes to make them go flat. The same is true for blues guitar, though there you are bending notes sharp rather than flat. For several years, I bent notes because it sounded good and didn't think too much about why. But the more I learned about Western music theory, the more mysterious the blues became. It's hard enough to understand how a minor third could sound so right on top of a major chord; but then why should it sound even better to deliberately play an out-of-tune minor third? It's not like every out-of-tune note sounds good in blues-based music. The Grateful Dead combine a lot of objectionably sour vocal harmony with Jerry's deliciously sweet bent notes on guitar. What's the difference, aside from intention?

I don't have a definite explanation of the blues' flexible use of certain pitches, and I certainly don't know the best way to teach this idea. My approach is to present students with specific blue notes from well-known songs and see what we can figure out. James Brown's "I Got You (I Feel Good)" from 1965 is an especially clear example.