Is there still an obvious residue there which you can feel
if you run your fingers over it, or just visible staining?
If the latter, it may be that the enamel surface had already
been damaged before the sugar was spilt on it.

I am pretty sure its just visible staining. It's black like charcoal.
and I can scrape it off carefully with a razor blade but I would
prefer to find a chemical method that may be a little less likely to
scratch the enamel.