The Black Jackets are a division of the Queen City Police Department. They handle investigations into supernatural crimes, whether that mean superpowered, magical, or something else beyond the realm of the normal. None of the Black Jackets are themselves superpowered.
They are so-named because they all wear long black leather trenchcoats and hats. These are not mere decoration, but actually function as magical armor. Dr. Darke placed magical spells on these trenchcoats for the police to act as wards and protect the wearers from low-level magical attacks. They also protect from knives, bullets, and other simple weapons.
Here is the opening of the Blackjacket story, “Devil Dog Day Afternoon.”