The most common reference to "triple black" with classic cars usually refers to a convertible vehicle with black exterior paint, black interior and a black convertible top. Other vehicles can be called "triple" as well, like my 1970 Mach which is Raven black with black Mach exterior decals and a black interior. Here is an excerpt from the Marti report:
867 70 mach 1's were painted Raven Black (about 2%)
626 of those had black hood stripes
365 of those had black interiors and were "triple black" cars