Black crud is actually very dark green crud more than likely. It is the antihalation coating that Ilford uses rinsing off after presoak, a very normal and desirable thing to have happen.
If the frame numbers are dark and readable on the film, then somewhere:
a. exposure was lacking on the film
b. developer was exhausted, or overdiluted, or just plain not long enough
If the frame numbers are light on the film:
a. see "b" above