[paphioboy]

Thanks for chipping in, nepnut... But I still think its some kind of infection. The affected black area is soft and mushy when cut and has a slight medicinal odour (like rot caused by Erwinia bacteria). If it were sunburn, I would expect the affected area to not spread and be dry instead of wet. I was quite worried by the speed of spread, so to be on the safe side, I cut the plant to prevent it from reaching the main stem.