Brief Answer:
It might be due to a respiratory infection and inflammation.

Detailed Answer:
Hi...
Welcome to "Ask a Doctor" service.
I am Dr Ajeet XXXXXXX

I have gone through your query and here is my opinion.

Please share a few details of your symptoms like since how long are you having a black sputum.
Is there any associated symptom like fever,shortness of breath etc?
What is your age and do you have any comorbidities?
These details would help me to evaluate you further.

A temporary discolouration of phlegm might be due to exposure to environmental pollutants and smoke.

A persistent black sputum might be due to a respiratory infection and inflammation.
At times it might be an indication of a more serious condition like lung cancer.

Waiting for your revert.

Stay healthy and stay blessed.
Regards.

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