That's because you made the most common porcelain teeth. After a long time, the oxidized gums will turn slightly black and occasionally allergic, which is normal.

I don't know if your doctor told you about this defect when you had a porcelain tooth examination. For example, a slightly better titanium alloy porcelain inspection will not happen. Of course, many other porcelains will not have black gums or allergies. Don't worry, wait until your dentist comes to work.