It may just brush off, with a brass or bronze brush ? but do make sure the brush really is brass or bronze.
A ?brass? brush I bought from a DIY outlet recently turned out to be brass-plated steel.
Fortunately, I discovered this fact before I did any damage with it.
The trick is to test with a magnet.
Alternatively, a soak in a very mild acid usually does the trick ? vinegar, a citric acid solution, or even Coca Cola ? followed by mild brushing.
The longer you spend on surface preparation and de-greasing before re-blacking, the better the result will be.