Black???? There were black hull Hunters?Nope! Just Black & White, then they got wild and used blue stripes/logos!
I found the same thing with my 1982 Hunter 30 when sanding the bottom. It's blue from the waterline down and white above. On closer examination it appeared to me to be either an original barrier coat or a thin top gel coat (why I don't know). There is white gel coat under the blue. No boot stripe in the gel coat.