Also if you can clean it up go to a scuba shop.
They have wet suit repair stuff called .... Aww crap I can't find the can I have.
But it's like a heavy duty black contact cement. Loses it's tackyness after a few hours.
Anyway I've used it on shift boots and such and it has seemed to work good. Had to build up layers though. And the boot was in the house for a couple weeks as I did that.
So maybe not a good solution on a CV, results may vary.
But I thought I'd mention it.

I know I have a cracked one on one side that has flung a bit of grease.
Probably going to just have a new axle on hand though as well as the stuff to do the wheel bearings and such and just do it all at once though once it gives me grief.