This anecdotal only: Most 'E' series kdx'ers agree that a '9' is wrong and an '8' is much closer to what it should be.
Don't rejet to an incorrect heat range plug. You're wasting your time.
Considering your bike to be in mechanically sound shape there is NO reason for it to be fouling the correct spark plug. No reason to not run the same plug for over a year regardless of what type of riding you do.
Your reference to the plug being 'black', but with other type of riding 'perfect' brings the assumption that you are looking at the color of the plug to tell you something about whether or not the air/fuel mixture is correct. Plug color doesn't tell you much of anything about that.
Take a read here:
!click this! to get a headstart on plug reading; what it is and what it isn't.