hello and welcome to HCM forum Christopher,
according to the history presented by you, molars are getting black from inside right? you have not mentioned about sensitivity to hot or cold
or even pain, i assume you do not face any of this.
Now,reason for a tooth getting black is either due to stains or due to caries, to prevent progression of caries,kindly visit your dentist
and get your teeth restored, but if stains is the problem then get oral prophylaxis(scaling) done for your teeth .
besides this,you should use a soft bristles tooth brush with an anti-cavity tooth paste twice daily.
maintain your oral hygiene
to have a healthy mouth.
i wish you good health.