The 'stiff cardboard' is likely made of layers of paper, impregnated with phenolic resins. Not a whole lot different from the stuff Formica counter tops are made of.
I believe the black 'papery' material is likely vulcanized fiber or slang; "fish paper."
I've found that the stuff is only marginally water resistant, loosing its insulating quality with exposure to water or high humidity.
See:
www.jjorly.com/fish_paper_die_cut.htm 
