Black stuff is likely
It is possible your compressor is overheating and the shell is oozing its bonding/sealing gasket material becaue of the heat. The lubricating oil could also be leaking out a bad gasket. It is either failing on its own, or a continuous call for cooling (stuck thermostat or dirty coils) is making it run too much. There is usually a thermal limit switch built in to the compressor which is likely bad in addition to the other things. (This would allow it overheat).