A pentatonic scale is a major scale with the two awkward notes removed. That is all it is, and it is why it is so forgiving: the notes most likely to clash are simply not there.
| Scale | Degrees | In C |
|---|---|---|
| Major | 1 2 3 4 5 6 7 | C D E F G A B |
| Major pentatonic | 1 2 3 5 6 | C D E G A |
| Minor pentatonic | 1 ♭3 4 5 ♭7 | C E♭ F G B♭ |
| Minor blues | 1 ♭3 4 ♭5 5 ♭7 | C E♭ F G♭ G B♭ |
Drop the 4 and the 7 from a major scale and you have the major pentatonic. Those two are the notes a semitone away from a chord tone, so removing them removes the friction. The minor pentatonic is the same five-note idea built on the minor third, and the blues scale is that with a ♭5 slipped in as a passing note.
Up major, down minor
Blues playing does not pick one. The trick, and it is the single most useful thing in this page, is to ascend with the major pentatonic, descend with the minor, and resolve to the major third. Most players believe a song is either major or minor. Blues is both at once, and that ambiguity is the sound.
Put both scales together and you get eight notes: 1 2 ♭3 3 4 5 6 ♭7. The app calls it the blues blend.
That adjacent pair is the whole thing. The ♭3 leaning into the 3 is the blues move, and it is why this scale sounds like the music while a plain minor pentatonic over a major chord sounds like someone practising.
The one that is not the same
Worth being clear about, because the names collide. The app’s Blues scale is the minor blues, six notes with the ♭5. The Blues blend is the eight note major-and-minor hybrid. They are different scales for different jobs: the first is the gritty one, the second is the one that follows the chords.