One of my favorite ground covers is Dwarf plumbago (Ceratostigma plumbaginoides). It is not picky about soil, sun or water conditions and will thrive on a hillside. It has glossy green leaves that turn bronzey-red in fall. In late summer, it blooms true-blue, although I have seen flowers in Zone 7 as early as July. The problem is, it's semi-evergreen; the leaves do disappear in winter. I leave the red stems with their burr-like seedheads on throughout the year. You could interplant with taller Artemisia x 'Powis Castle' for year-round greenery. This Artemisia thrives in clay soil.