Black death is a description of having algae, or more accurately, fungus in the diesel fuel. Creates a black sludge that plugs filters, lines and such. Pain in the tail to get rid of. Black death has nothing to do with fumes, so the rest of your post is unrelated to it.

As for smelling fuel, any leaks or drippage can linger odors for a very very long time. Might be useful to wash out under the bonnet and see if it returns. A clean engine bay will make it easier to detect the source of any leaks.