Black Death, as it is known by A/C techs, is a result from the breakdown of the refrigerant-lubricant. This causes excessive wear inside the compressor and the excessive ware causes a black residue that is actually a combination of solder flux from the condenser and aluminum shavings from the old compressor. When this goop cools, it hardens in the condenser. Repeated flushing will not remove it, however, when the system is charged and running it liquefies, flows out into the liquid line and clogs the orifice tube. With less refrigerant flowing, past the orifice tube, the compressor is starved of lubricant and will lock up. The only answer is to replace the condenser.