Thank you for your question and welcome to Just Answer.
The black color you see on the tooth with the cavity is one of two things:
1, The decay itself is showing through the side of the tooth. The decay has grown internally into the tooth and is visible through the enamel shell of the tooth.
2. The nerve of the tooth is dead. A dark tooth indicates the blood vessels in the nerve have ruptured, causing internal bleeding of the pulp. The dark color is similiar to a bruise. In this situation you need to have a root canal to save the tooth.
Nothing you try to do at home, including using mouthwashes will get rid of the problem and you will have to see your dentist to have the decay removed in the case of #1 above or have a root canal or extraction in the case of #2 above.
I hope this answers your question. If you would like to discuss this further or have any additional questions, please reply to my answer and I will get right back to you.
Sincerely, XXXXX XXXXX DDS
Thanks for the additional information. Most people have to have their wisdom teeth removed at some point (usually in their late teens) so this is not unusual in your situation. Dental decay unfortunately is painless until it gets to the stage where a root canal is most likely needed. Most people unfortunately don't seek dental care until there is something painful in their mouth or start seeing the darkness of advanced decay. This is why we advocate at least yearly check ups and cleanings, in order to look and catch some of these problems early, when a simple filling can solve the issue. THere is no correlation between childbirth and dental problems, but it is a fact that the gums can become quite inflamed and swollen during pregnancy, which in turn can initiate periodontal disease, which is the situation where the bone around your teeth dissolves away. This can create spaces between the teeth where food and bacteria can harbor and cause decay. This might be the situation you are experiencing, but without seeing your mouth, I can not tell. The best person to answer this issue for you is your regular dentist, who knows your specific mouth and circumstances. I hope this helps.