Is this mold or just black wood?

That is mold. Assuming this is what it looks like after it's been sufficiently dried so more mold won't develop beyond what we're seeing here, that is surface mold isolated within a wall cavity, and you've got nothing to worry about.

Typically, if it is surface mold or the decayed portion of the wall system is small enough to be safe and acceptable, you would leave it alone, as molds become dormant below 20% WMC (your framing should be between 12% and 16% once fully dried and ready to be closed back up), as long as the following criteria are met:

• If the sheathing was never saturated (or above 28% WMC) or if the saturation was limited (in a non structural wall);
• If the moisture problem has been remedied (dried and not going to get wet again); i
• If it will not be disturbed, so as to spread spores into living space; and,
• If the decay is surface only (has not transmitted through to the other side of the sheathing is a good indicator of compromised sheathing).

Best practice would be to replace the sheathing in the event any of the above are not met. Cost of doing the repair vs cost of not doing the repair will drive the decision, except in the case of damage to a critical building component in which case the cost of not doing the repair is also a question of safety/liability.