This usage has leaked southwards via the fact that neither "treacle sponge" nor "treacle tart" contains anything black: the syrup in them is golden.

Not where I come from. I've never come across any dessert involving syrup being referred to treacle pudding or tart, only syrup pudding or tart. And similarly all the treacle puddings and tarts I've ever encountered have had treacle in.

Are you sure you're a real southerner?