We have found 12 plants that meet your criteria.

Price Pruned
Granny's Bonnet ‘Aquilegia vulgaris var. stellata Black Barlow’ Striking dark purple, double flowers NOW £4.95 Buy
Day Lily ‘Hemerocallis American Revolution’ Large, deep blood-red flowers in summer £5.45 Buy
Intermediate Bearded Iris ‘Iris Langport Flame’ Sumptuous purple and orange fragrant flowers £6.95 Buy
Black-eyed Susan ‘Rudbeckia occidentalis Green Wizard’ Bright green sepals with black centres £5.95 Buy
Price Pruned
Bearded Iris ‘Iris Superstition’ Purple-black flowers NOW £5.95 Buy
Price Pruned
Beard Tongue ‘Penstemon Raven’ Dramatic, deep purple, foxglove-like flowers NOW £5.45 Buy
African Lily ‘Agapanthus Black Pantha’ Deep blue, almost black flowers £8.95 coming soon
Price Pruned
Granny's Bonnet ‘Aquilegia vulgaris William Guinness’ Unusual purple-black flowers NOW £4.95 coming soon
Bearded Iris ‘Iris Before the Storm’ Extremely dark, nearly black flowers £6.95 coming soon
Intermediate Bearded Iris ‘Iris Langport Wren’ Dark magenta flowers. Very free flowering £6.95 coming soon
Pincushion Flower ‘Scabiosa atropurpurea Ace of Spades’ Gently fragrant they make good cut flowers £5.95 coming soon
Harvington Hellebore ‘Helleborus x hybridus Smokey Blue’ Unusual, dark purple flowers with a deep blue sheen £9.95 coming soon