We have found 16 plants that meet your criteria.

Price Pruned
Granny's Bonnet ‘Aquilegia vulgaris var. stellata Black Barlow’ Striking dark purple, double flowers NOW £4.99 Buy
Dusky Cranesbill ‘Geranium phaeum’ Small, deep maroon flowers. Good for groundcover £5.99 Buy
Price Pruned
Dusky Cranesbill ‘Geranium phaeum var. phaeum Samobor’ Small, purple-black flowers. Good for damp or dry shade NOW £5.49 Buy
Tree Peony / Tree Paeony... ‘Paeonia delavayi’ Tree peony with deep, maroon flowers £16.99 Buy
Moor Grass ‘Sesleria autumnalis’ Perfect for informal edging £5.99 Buy
Trillium ‘Trillium cuneatum’ Big maroon petals £3.00 Buy
Trillium ‘Trillium recurvatum’ Great for naturalising in woodland £3.00 Buy
Granny's Bonnet ‘Aquilegia vulgaris William Guinness’ Unusual purple-black flowers £5.49 Buy
Price Pruned
Clematis (Group 3) ‘Clematis Black Prince’ Beautiful deep purple flowers NOW £7.99 Buy
Price Pruned
Clematis (Group 3) ‘Clematis Warsaw Nike’ Sumptuous deep purple flowers, NOW £7.99 Buy
Lenten Rose ‘Helleborus x hybridus Harvington Shades of Night’ Early spring flowers in shades of deep purple £9.99 Buy
Harvington Hellebore ‘Helleborus x hybridus Smokey Blue’ Unusual, dark purple flowers with a deep blue sheen £9.99 Buy
Price Pruned
Beard Tongue ‘Penstemon Raven’ Dramatic, deep purple, foxglove-like flowers NOW £5.49 Buy
Black-eyed Susan ‘Rudbeckia occidentalis Green Wizard’ Bright green sepals with black centres £5.99 Buy
Black Elder (syn. Black Beauty)... ‘Sambucus nigra Gerda’ Dramatic purple foliage £8.99 Buy
Price Pruned
Red Hook Sedge ‘Uncinia rubra’ Wonderful deep bronzy-red foliage NOW £5.99 Buy