There's some baked-on toner on the fuser roller
which might[1] (most
probably!?) be the culprit, but gentle cleaning doesn't seem to get rid of
it; question is, how aggressive can I be with it? I don't know if the
blue-gray color is just a thin coating of something magical, or if it's a
hunk of solid metal which can be scrubbed.
This is a teflon coating that should not be harmed. If you can't clean
it with scotchbrite, you can buy a new fuser roller. It will be some $10 and
your printer will work like new.
Try printerworks (google it), they have detailed catalogs and cheap
prices.