There are really dark Coleus, like
Coleus (Coleus scutellarioides 'Shiny Shoes')
Pennisetum/Cenchrus 'First Knight' is one the darker Napier-hybrid types.
Napier Grass (Cenchrus purpureus 'First Knight')
Pseuderanthemum atropurpureum 'Black Varnish' is pretty dark, too.
Pseuderanthemum carruthersii var. atropurpureum 'Black Varnish'
Cardinal Flower (Lobelia cardinalis 'Black Truffle') has darker foliage and red flowers.
For very early spring, there are blackish poppies, violas/pansies, and cornflowers. Viola/pansy plants can be planted in fall to overwinter. The really deep red Snapdragons, like
Snapdragon (Antirrhinum majus 'Black Prince') , would make good companions. Some strains have darker leaves than others.
There are some nice Dahlias with deep maroon flowers, or with blackish foliage and red or pink flowers. Choosing only heat-tolerant cultivars would be the key with Dahlias since they don't care for hot nights. You'd have to do your research.
