Last year we had a number of closed up houses that flooded and developed black discoloration on the lower portion of the basement walls. I don't call it mold until I have a mold report in hand. In any event we cut out the bottom 3 feet of drywall, wash it with swimming pool shock (bleach on steroids) 3-4 times 3-4 days apart. then we paint it with Kilz, and then replace the drywall with waterproof drywall and cover with wainscotting. No problems. The swimming pool shock was what a professional mold mediation company used on a 25K gut job. He mentioned chemically treating with something that sounded like the cure for leprosy. But the common name is swimming pool shock. Mixed with water and applied with a floor mop. Wear a ventilator, keep the windows open and a fan running, the fumes are nasty. Also, avoid any amonia near bleach. When mixed they form Chlorine Gas, outlawed by the Geneva Convention. Nasty stuff.
Now if the discoloration is such that it pops the wall board off the studs, or is inside the cabinets and such. I.e permeated the entire dwelling, hire a professional (not an arsonist) since that's beyond bleach treatment.
There are those that will disagree, but the methods I've laid out were as taught to me by the mitigation companies. Yup...I pay attention.