See what the directions say as far as what kind of metal you can use it on and the prep needed. It sounds like it may just be stove black used on wood stoves, but there are also products out there may for gun bluing or blacking.
Some products can only be used on Iron or steel. This type of blackening is actually a controlled for of rusting so it wouldn't work on brass. hope this helps a little

Make sure your fire doors are not covered with a sealant before you try to blacken it. If they are still shiney (and not brand new) they probably have some sealant on them and will not blacken anyway until the surface is stripped. Then there are any number of products specifically designed to darken brass- black, dark green.