A black draught was a saline aperient mixture used, along with a blue pill, as a purgative in the 19th century and well into the early part of the 20th century, with veterinarians prescribing these to constipated cattle and horses.
Mrs. Beeton's Book of Household Management has this recipe for a black draught:
2587. The Common Black Draught.-- Infusion of senna 10 drachms; epsom salts 10 drachms; tincture of senna, compound tincture of cardamums, compound spirit of lavender, of each 1 drachm. Families who make black draught in quantity, and wish to preserve it for some time2 drachms of spirits of hartshorn to each pint of the strained mixture, the use of this drug being to prevent its becoming mouldy or decomposed. A simpler and equally efficacious form of black draught is made by infusing 1/2 oz. of Alexandrian senna, 3 oz. of Epsom salts, and 2 drachms of bruised ginger and coriander-seeds, for several hours in a pint of boiling water, straining the liquor, and adding either 2 drachms of sal-volatile or spirits of hartshorn to the whole, and giving 3 tablespoonfuls for a dose to an adult.
'Black Draught' is also the name of a once-common commercial liquid syrup laxative, sold since the late 19th century, a cathartic medicine composed of a blend of senna and magnesia. Much like castor oil, it was a commonly used folk remedy for many ailments.
A musical jingle for the product was sung by a very young Dolly Parton: Smile from the inside out smile from the inside out Black Draught makes you smile from the inside out.