I advise you to visit a dentist because the black dot on the Gum due to any reason is not good sign. It can be hematoma or even gum cancer. Only dentist can give you differential diagnosis. Visit him and get treatment. But the oral cancer occur in old age. If you are below 24 and don't smoke then the chances of oral cancer are rare.
It could be some food left in your braces but visit a dentist to be sure it's not something serious