Is golden syrup with black around the edges bad? I had a tin of golden syrup for a long time - longer than I care to admit. When I opened it up yesterday (to sweeten up some mulled wine), it was granular (a bit like honey can be), and black around the edges. 

Is this 'bad', or just different? 

Is the blackness the result of some chemical reaction with the tin?

And in general, does it eventually go off, or does it last indefinitely?
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.