Welcome to the site meerkat!
I think I'm right in saying that Eric was never published with the black covers. It was originally a large format paperback with illustrations by Josh Kirby. A later, smaller copy then came out by Gollancz (rather than Corgi) without the illustrtions.
The really odd thing is that there seems to be a 'black' cover for the audiobook, but not the book.
Look, you've got it all wrong! You don't NEED to follow ME, You don't NEED to follow ANYBODY! You've got to think for your selves! You're ALL individuals!
Yes! We're all individuals!