meatnub wrote:

These older "metal" windows have black dirt in the window crevices.
There is black dirt in the window sill (between outer most window and
inside window).

Is this mold? Could it be full of allergens?

It's most likely just dirt that has accumulated in the sill between
the inside and outside windows. Leave the outside window open during
the rain, and sometimes the rain gets into the opening between the
outside and inside windows. Rain is dirty, and over the years that gap
between the inside and outside windows can accumulate a lot of dust
and dirt from the rain and wind.

My question is - should i take the time and get a tooth brush and get
into the nooks and crannies and remove all that black dirt? Or just
clean it normally with some cleaner and paper towel.

I just wipe it out with a wet paper towel during spring and fall
cleaning.