Well, it was probably not a cat in a darkroom, but a dusty/dirty darkslide or changing-bag leaving a hair on the film-surface at the time of exposure. That unexposed area on the film then makes the black line on the print.
Does ferricyanide attack that paint-on rubber-mask material? Might be worth a try to localise the effect a bit? Depends on the size of the area of course.
The alternative possible technique with knifing also depends on the specific area of background one has to match of course. Mr.Carnie is well aware of the options I think.