A delightful combination: White Comfrey (Symphytum orientale) and Green Alkanet (Pentaglottis sempervirens). They have rather similar leaves and among this mass of greenery are flowers of pure white and electric blue.
 They both grow wild and free in London and the home counties but I deliberately paired them in the garden. Some years ago I saw a long border at Kew which had been given over to a mass of S. orientale and P. sempervirens. I don't know if it was by accident or design but the effect was stunning.