Campanula lactiflora (Milky bellflower):
Clusters of blue flowers on tall stems in mid-summer. Ideal for filling out borders and adding height. Fills out quite quickly too. To 1.5m. Hardy.
Caulophyllum thalictroides (Blue cahosh):
Shade-loving with yellow-brown flowers in spring, followed by blue berry-like seeds in autumn. Attractive, divided blue-grey foliage. Hardy. Up to 1m.
Leucanthemum x superbum 'Wirral Supreme':
Tall form of Shasta daisy with white flowers and multiple shorter petals around the golden centre, making an attractive frilly effect. Hardy. Up to 1.5m.