Is this 'black stuff' shiny, and hairless spots on the T? might just be where the hair has rubbed off the exoskeleton, as is common in almost all wild-caught tarantulas. Did you just aqcuire this T or is this a long term captive with a recent 'black stuff' spot? Give us the history of this animal, someone may be more likely to help.
i got him wild caught and he developed this black smudge a few weeks ago it hasn't molted yet in my care. and it won't gain weight. its ate around a dozen crickets but not even the slightest growth in the abdomin.