I'm doing a bit of a "thinking post" here, and am putting down my thought process since there's a chance, albeit small, that my random thoughts might help someone figure it out.
Based on the hint,
It is impossible to make a black koan by adding extra letters to the end of a white koan.
I've made an abridged white koan list.
Because of this hint, knowing, for instance, that PINE is white means that PINEAPPLE gives no extra information, so my abridged list includes PINE but excludes PINEAPPLE.
I do not guarantee I didn't accidentally miss one.
BANA
BAND
CAKE
EARS
MELON
NANANA
PINE
PIRATE
RATES
The bolded koans will become black if their last letter is removed.
Also based on the hint, I can conclude that the following koans are black (this is probably not helpful information):
because, if any of them were white, any koan starting with them would have to also be white. Generalizing this list,
the first letter of any black koan is itself a black koan
Me rephrasing the hint in my own words in the hopes that it might help me think:
A white koan cannot under any circumstances become black by adding more letters, but a black koan can become white by adding letters.
Based on my abridged koan list, I will try to determine what the shortest form of each white koan is.
EA
MELO
MEL
ME
NANAN
NANA
NAN
NA
PIRAT
PIRA
PIR
RATE