If you pressure washed it and it is still dark pour laundry bleach on an
area, if it lightens in a few minutes its mold. If its mold use a garden
sprayer with 50% bleach, or mop it on let it sit for hours, then
powerwash again, if its mold do your basement walls also . You may have
a high water table keeping the floor damp, a dehumidifier would be best,
if its mold.