I think the difference is the look on the razor in yer hand...

Black rust looks like "rust" it has bumps and stuff, craters, pits, etc:

Devil Spit is a slick smooth deceiving looking stain from the pits of hell, that really looks as though you could just use MAAS and it would come right off....
BUT NOOOOOOoooooooo this stuff just gets worse the harder you try...

I guess it would technically be called a "water stain" ???????? I liked Devil Spit, and no it is not a Glen original I know I read it someplace....