The reason why your tooth is black inside is because of a condition called pulp necrosis which is the death of your tooth's internal tissues.
The pulp necrosis or death of your tooth's internal tissues can occur due to severe tooth decay, infection or trauma which can lead to the release of the dark pigments which stain your tooth black.
Having your tooth turn black inside is not normal and you should see a dentist.
The pulp that leads to pulp necrosis is located within your tooth and contains blood vessels and nerves.
And when the pulp in the teeth die and leads to necrosis it releases pigments which stain your dentin which is the layer below your tooth enamel and often colors the tooth a dark or blackish color.
Causes of pulp necrosis include severe tooth decay, trauma and infection...
Bacteria can enter the pulp through cracks or cavities in the teeth and cause an infection called pulpitis and can eventually lead to the pulp necrosis forming.
A blow or injury to your tooth or teeth can also damage the pulp and lead the pulps death and pulp necrosis and black teeth.
And if you have cavities which are left untreated, the cavities can also penetrate deep into your tooth and reach the pulp and cause infection and death of the pulp.
Other potential causes of pulp necrosis are tartar buildup, medications such as tetracycline antibiotics and old dental fillings.
If you have a black tooth you should see a dentist promptly for diagnoses and treatment and if the pulp is dead you may need a root canal to remove any dead tissue and prevent further damage.
And you may also need fillings, crowns or even a tooth extraction ins some cases.