Does anyone know if it is normar to find a skim layer of "black sludge" at
bottom of tank and directly on top of all filter media at end of season
with
a Skippy Filter. I found approximately a 1/4 inch of sludge which did not
totaly "stink" but did have a slight hint of decomposition by anaerobic
bacteria.

This is normal for a bio- filter, my bio filter does the same thing all year
long and I back flush it as needed ( once or twice mid season) - the sludge
is a bi product of the bacteria process in the filter. In the fall, I wash
the media out as well as possible and hang it up to dry before I store it
for the winter - ready for next spring
Gale :~)