"If it's a small area, you can use light-colored children's molding dough to press onto the brick and lift the soot away. Or dip a scrub brush into a pan of full-strength white household vinegar. Scrub a small section at a time, working up. It may take a lot of elbow grease and several applications to remove all of the soot.
Caution: If your fireplace brick is old and crumbling, test the molding dough in an inconspicuous spot -- you don't want to pull off any surface material. To remove soot from brick that is not in good condition, mix a 50/50 solution of laundry bleach and water, pour into a spray bottle, spritz the bricks, then scrub with a soft-bristled brush. Rinse with water"