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.
cooking.stackexchange.com/users/1549/Chris Steinbach
My favorite saying is When in doubt throw it out .
Having said that you should throw that away.
The black can either be from the oxidization of the tin or it could be mold growth.
In either case golden syrup only has a shelf life of about two years; or one year after being opened, and it should be refrigerated after opening.
The granulation could be fixed by reheating but the black is not worth the risk.
cooking.stackexchange.com/users/2283/Varuuknahl
Golden Syrup pretty much does not go off.
It is a blend of sugars and preserves as well, it should keep for many years.
It will crystallise slightly over time though that can usually be solved via warming.
Bacteria is highly unlikely, mould can form on it - that would be a reason to chuck it out, possibly not all of it.
The only serious issue is from the tin itself corroding, which is what the black area could be.
I've known households to use that stuff from a 20 year old tin and have no issues whatsoever.
It definitely does not need refrigerating any more than sugar does.
cooking.stackexchange.com/users/3432/Orbling
I've just thrown away an open tin of the syrup, as I thought it had a funny aftertaste.
Still within the sell by, but open for about a year.
cooking.stackexchange.com/users/7881/karen
Sweetheart, if you have doubts- just chuck it!It's not worth a couple bucks to eat something that you think will could make you sick full stop.
cooking.stackexchange.com/users/17348/lese
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?
Is golden syrup with black around the edges bad?
cooking.stackexchange.com
<storage-lifetime><syrup>
cooking.stackexchange.com/questions/9822
cooking.stackexchange.com/users/3680/Benjol
Golden syrup is said to improve with age, but the tin might corrode and this could have caused the black area. If in doubt, it may be best to throw it out. Golden syrup doesn't really go off unless the tin has started to corrode.
Golden syrup doesn't go off and improves with age. If the tin has started to corrode then you should throw it away. Golden syrup has a shelf life of about 2 years.