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.
“Men never commit evil so fully and joyfully as when they do it for religious convictions.” – Blaise Pascal