Black Horehound and White Horehound are plants in the same family, Lamiaceae, but different genera.
Black Horehound Ballota nigra is a plant of hedgerows, waysides and rough ground, and is common in most of England and Wales, including our patch here in North-west Norfolk.
White Horehound Marrubium vulgare is a plant of bare, dry places on limy soils. It is sparsely scattered in the British Isles, becoming much rarer, and is probably only native south of a line from West Norfolk to Denbs. Each year a few plants usually can be found at Ringstead Downs. On Tuesday I noticed that some of the plants were in flower and today I managed to take a few photograhs. (Some Catmint Nepeta cataria plants, not yet in flower, were growing with the White Horehound.)
Black Horehound Ballota nigra
White Horehound Marrubium vulgare