Paddy grain become black on at the time of harvest, what is the reason (deficit)behind it

It's false smut.False Smut .

Dear Bhagabana Jal It is false smut. Since the crop has already matured , any efforts to control the disease may not be of much use. To control the disease, copper oxychloride 3 gms in one litre of water may be sprayed at the panicle initiation stage. If the disease incidence is more , take up control measures as given above during next crop season. Good luck.

Use slogan or galileo sense

Use fungicide when crop get. Upto 10% fruiting