I am really not sure how modern rappees (which Best Dark is) are made, but some snuffs (including some rappees) of bygone days were dyed using the combination of natural dyes (like logwood) / tannins and mordants, which contained sulphur compounds. However, this method was probably applied only when snuff was made from inferior tobaccos.
I think that “snuff hangover” might be caused by the hazardous constituents of dark fired tobacco (mainly PAHs) and (perhaps) to some extent by the byproducts of fermentation (which might increase TSNAs content). Just a thought.
I’m not a fan of Bernard and WoS dark snuffs. Much prefer SG offerings to them, for I find them much easier to take and more satisfying in nicotine department. Black Magic (Silky Dark/old Viking Dark) is my snuff of choice, when it comes to the dark side.