The brick above and around the fireplace opening can sometimes be stained by smoke puffing out of the fireplace opening. Unfortunately, brick is porous and the stain will often be quite difficult to remove. You should be prepared to clean the entire brick front since any cleaner strong enough to remove stains from brick will also remove all buildup of normal household dust or dirt from the brick. Cleaning only the dirty portion of the brick will leave the remaining portion with a different appearance.
Several cleaning methods are:
Commercially available brick cleaning products, be certain to follow the instructions carefully.
Oven cleaners - some work well.
Muriatic acid - be certain to protect all surfaces and furnishings other than the brick, wear gloves and goggles, and completely rinse any acid residue from the brick.