Pickups that are potted usually have a very hard coating that is used to seal the coil and components usually within a cover. Epoxy is also poured in rubber molds and the electronic components are covered with the liquid and let dry. It is done to seal the pickup parts to keep them from being tampered with and to protect the sensitive electrical components especially in pickups with active circuits. I’ve seen black and other colored epoxy used on Gibson humbuckers during the 70s and 80s. I’ve seen problems with epoxy too. As some brands of epoxy harden, it can shrink and break the fine magnet wire in the coil. The bobbins and hookup wires should be protected from the epoxy solution so it doesn’t damage the coils. Epoxy when it is mixed is pretty think and can’t permeate into the coil to reduce microphonics. It is normally used to hold firm the other components such as magnets, pole pieces, bobbins, ferrous spacers and to keep them from vibrating. It the parts vibrate when playing loud, then feedback can occur. It is difficult to work on pickups when they are epoxied and damage to the bobbins and coil can occur if you try to pry the epoxy off.