They still could be mildew stains. Try scrubbing them with clorox to kill the mold or mildew.
If they are on a wall, you could try sealing it with block sealer, they make a primer called KILZ that is suppose to help prevent the return of such stuff, then paint with a paint that you add mildew inhibitor into. But first try full strength clorox and see how it reacts. Be sure to have some ventilation.