Wow, that was completely not in the least bit funny. They shouldn't even tried it if they can't say the word "fuck". Using a nonsense F word just takes the humor out of it. If Fallon wants to work blue than he should work blue; if he wants to work clean he should work clean. Mixing the two in this day and age doesn't work and comes off as lame.
EDIT: I watched the interview and apparently that was Yoko and Sean's idea for the show. Instead of being star struck and trying to sniff John's proverbial jock strap he should have told them no.
EDIT (again): Fracking is apparently a thing. Still, Dylan's career is rolling over in its grave.