Phlox ‘Blue Paradise’ is among the bluest! Another one of the so-called ‘changeling’ phlox, whose flower color changes markedly according to differing light conditions. The flowers are a medium purple by day, but change to a bright violet-blue at dusk and dawn, and during cloudy days. In these conditions, it is the closest to true deep blue that I know of. Planting it in part shade will cause the deep color to linger longer through the day. Introduced by Piet Oudolf in 1990.