Diminished tuning:

C E♭ F♯ A
and draw notes follow a repeating sequence of

D F G♯ B

If you just blow and draw, nothing sounds good. In Diminished Tuning, you have to be conscious of your scale pattern, and you’ll have to play lots of half-step bends.

http://www.wildflowerharmonica.com/diminished-tuning/

https://en.wikipedia.org/wiki/Diminished_tuning