Cirsium rivulare 'Atropurpureum' / RHS Gardening. This is a great plant for the back of a border and it has a long flowering period. Also bees love it!
aster lateriflorus "Lady in black" pink flowers from mid - summer to mid- autumn and dark foliage.
persicaria amplexicaulis taurus
Sedum 'Purple Emperor'. Stunning = everyone notices it. Flowers from mid-summer. Provides beautiful winter structure.