Hi. As you said you are brushing thrice a day you take best care of your oral health still you develop a black spot which makes you feel bad don't have to be afraid it is not paining yet for prevention of caries your revaluation is important every six months.
Next Steps
Get clinically evaluation from nearby dentist
Health Tips
Start using listerine anticavity mouth rinse before sleep and fluoride varnish can applied by professional for better cure. Regards. Drpuneet kumar.