I just ripped up my ~ 10 year old carpet in the living room and
hallway. In many areas there was a black gunk stuck to the hardwood
floor under the pad. It seemed to be confined to the high traffic
areas. It also was confined to the center portion of the hardwood
floor boards. Is this mold? The carpet has had a few dog accidents
over the years, but the areas were too large to be strictly from this,
plus the hallway seemed the worst and I don't recall ever having to do
any clean up there. I'm thinking I may have let it get too wet at some
point using the home carpet cleaner (bissel upright). Or is this just
a byproduct of the pad, dirt, dust, etc.?

I mopped up with spic & span plus bleach. I'll give it a second
cleaning tonight before the new carpet goes in tomorrow afternoon. The
new pad will have the waterproof membrane on top, so hopefully that
will minimize this in the future, if it is really mold I am seeing.