Once I have a blackbelt, is it too late to become an uchi-deshi?
No, I don't think so in general. The important thing will be to put aside what you already know, and try to start as if from scratch. Keep what you've already learned kind of segmented off, and push yourself to embrace what is right in front of you.
The Yoshinkan offers a senshusei course, which is about as close as you can get to an uchideshi program of sorts. For the course, no matter what your rank, you wear a white belt. From what I understand, yudansha who go through the course prize the rank they earn in the year to 2 years they spend there very highly.