Color /ink migration may or may not be an indication of vinegar syndrome decay for the paper. Best case scenario is the binding or fixative agent applied to the back cover has degraded to the level where it no longer prevents the ink from rubbing off. Modern adhesives and additives to glossy paper stock should not contain acetate-based compounds but this varies from the era of printer and paper supplier of course. Hellblazer was printed right at the edge of the era (late 80s) when publishers began experimenting with alternative papers (e.g. company name Mando, Baxter, Hudson papers) If you don't smell any vinegar, I wouldn't worry too much. I would just replace the backing board with a fresh one. Many collectors swear by the use of micro chamber papers to limit the effects of vinegar syndrome. I can't vouch for them but it may help if you have books that aren't too far gone.