I just had a cavity today morning and now after brushing nicely, the cavity has got a little less black. does that mean my cavities will go away ?

It would be unlikely: Cavities only get bigger with time and if you wait too long your tooth nerve will get infected and then you are going to need a root canal and eventually a crown when you get a little older. It is much better to have small cavities filled than to risk the alternative. Go to the dentist for exam, x-rays and after proper diagnosis, treatment as needed.

Chipped: It sounds like your tooth may have chipped or fractured. Your cavity is still there and will need treatment.

No: Cavities don't go away just because you brush them. Maybe it was just a stain or plaque you brushed away. See a dentist to make sure.

No, it is oxidation: Not all cavities come with stains and not stains are cavities. But stained and darkened areas on teeth are susceptible attack points for cavities. It destroys tooth structure the way rust oxidizes metal. No amount of brushing or polishing will ever remove the cavity once it has taken control. No matter how much 'black' you remove from tooth brushing, the base problem remains until treated.

Need filling: Cavities are bacterial infections. The bacteria will produce acids that destroy the tooth. Cavities cannot be removed by brushing alone. You need to see a dentist to have the decay removed from the tooth and a filling placed. Remember that the longer you wait to get treatment, the bigger the decay gets and the more difficult and costly treatment can be.

Sorry but no: You might have had a little food debris in the cavity and brushing may have decreased the darkness but decay does not "go away" if you wait too long you may need a root canal. Which can be 5 times more expensive than a filling. Get it filled soon.