There are different types of treacle, Black is only one, Golden Syrup is a lighter-coloured type of treacle and often substituted with corn syrup in recipes where black treacle is not available. In Britain, the main producer of black treacle (and Golden Syrup) is sugar refining company Tate and Lyle.
Roasted grapes are sophisticated. Toss grapes with olive oil, salt, and pepper, and roast them on a sheet pan for about 30 minutes at 425Â°F. The grapes will get blistered and caramelized on the outside, and extra juicy and practically jammy on the inside.
Put the mashed grapes into a large stockpot. Slowly heat the grapes and juice to a simmer on medium heat and then simmer for 10 minutes. Stir occasionally so that the grapes don't stick to the bottom of the pan. Halfway through cooking mash some more, breaking up as many of the remaining grapes as possible.