I competed a challenge run with a joker I've never used before but it didn't unlock. The undiscovered jokers can be unlocked during an unseeded run. Also the challenge was "Mad World".
#Challenge runs don't unlock jokers!
onyx tangle
marsh shell
This is intended, you cannot unlock things through challenges
The condition to be able to unlock things is that it's unseeded AND a normal run
onyx tangle
The problem is is that it doesn't specify it
Bilatro is a very legalese sort of game
Also, I don't think the challenge runs are seeded
marsh shell
challenge runs aren't seeded but that's why i said the condition to unlock things are that the run has to be unseeded AND normal