Are you sure the chrome hasn't been discolored by heat?

The rough black stuff should come off with just about any decent cleaner.

If you are down to smooth metal finish and still have a discoloration, it could be from the heat of the fire and that is likely there to stay, short of re-chroming.

As a last ditch effort, you could try an acid based metal brightener like they use at commercial truck washes.

Kevin