A. Henri de Marne responds: The black discoloration is probably a mold. This is a surface mold; it does not have roots that penetrate into the wood, but it is a precursor to true rot. If the wood is still solid, you should be okay if you treat the lumber with liberal doses of a copper-based wood preservative, such as Cuprinol Green (Cuprinol Products, a division of Sherwin-Williams; 800/424-5837).
As for the sheathing, if it is truly waferboard and not OSB, it is not a structural panel. However, it might be all right for a porch wall. Cut the damaged sheathing back to good wood, and replace it with pressure-treated plywood after you have treated the framing with the wood preservative.
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