chillies

Some chillis and peppers have a black stage between green and red, as they ripen.

Is this what is happening to yours?

Sounds daft, but are they

'Soft and squishy and black'

Or

'Firm, solid and black'

As that can help you tell between good black and bad!
Squishy is never good- and points to a blossom end rot type issue. 

Also, how's the rest of the plant?

Good luck.