Those appear to be socket head cap screws that are often made from black oxide alloy steel. The black oxide process is used to add,
"mild corrosion resistance, for appearance, and to minimize light reflection. To achieve maximal corrosion resistance the black oxide must be impregnated with oil or wax."
Moisture in the air (humidity) can contribute to corrosion. Humidity present in lower levels of conventional homes (i.e., in basements) can condense on cooler surfaces. Increasing air flow, increasing temperature, and operating a dehumidifier all help reduce problems related to humidity and condensation. However, even mild condensation over years could have contributed to the oxidation you see, it is not uncommon on those types of screws.
If you are concerned, open the top and look for any obvious evidence of corrosion inside (be safe - unplug the amp first and do not touch any large capacitors or other areas which could result in shock).
If you see significant evidence of corrosion inside, take a picture and send it to CJ for discussion.
However, if everything looks fine then I wouldn't worry about it except to make sure you are operating the amplifier in a warm, dry place. If you wish, you can replace the screws or clean and oil them as discussed by
@russ69.
