Is golden syrup with black around the edges bad?
Golden Syrup improves with age according to Heston Blumenthal's "In Search of Perfection". Here he analyses a 70 year old glass jar of Tate and Lyle Gold in the lab finding that the "older treacle contained the same flavour compounds as the normal stuff, but each was intensified".

In his recipe he fakes the ageing process by baking the tin at 70ºC (158ºF) for 24 hours noting that the flavour will continue to improve up to 100 hours.