Ajuga reptans, or Bugleweed, a slow-growing perennial ground cover and member of the mint family, is one of the darkest Ajuga's I have ever seen.  Unlike its wayward mint relatives, it can be kept in check, but it would probably also look very nice in a large container or trough garden.  The scalloped, deep green leaves, sometimes appearing almost black, are tinged with bronze and purple and are attractive in their own right, but in spring, there is another matter to consider: 10-inch spikes of shimmering blue/violet flowers that take it over the top.  Ajuga reptans (Bugleweed) appreciates partial sun/full shade and moist, well-drained soil, but it is adaptable to harsh conditions.    Zones 4-9.

This cute Ajuga from Valfredda, Italy has chocolate/bronze foliage, which is elongated rather than round, forming a low-growing, dense mat that is ideal for a small garden.  Not only that, but "Valfredda" has bluish purple flowers on 3-inch stems to complete the picture. Ajuga  appreciates partial sun/full shade and moist, well-drained soil, but it is adaptable to harsh conditions.   Zones 4-8.  Quarts.