R. Sinclair Black obtained the degree Master of Arts (MA) at the University of Aberdeen, Scotland, in 1885. He then decided to take up medicine and qualified as Bachelor of Medicine and Master in Surgery at the same university in 1889, while completing the Diploma in Public Health during the same year. He remained in Aberdeen for some time as a demonstrator in zoology and then worked for short periods at several institutions: as assistant medical officer at the South Western Fever Hospital in London, the West Riding Asylum in Yorkshire, and an asylum in Lancashire, and as acting medical superintendent of the Small-pox Hospital at Dartford. He became a member of the British Medical Association and a fellow of the British Institute for Public Health.
In May 1894 Black was appointed medical assistant at the Colonial Bacteriological Institute in Grahamstown, under Dr. Alexander Edington*. During the rest of that year and in 1895 he experimented with the possible transference of horse-sickness to sheep by inocculating them with the blood of infected horses, eventually concluding that the disease did not affect sheep. During 1895 he also investigated the pathology and bacteriological diagnosis of diphtheria and read a paper on this work in Grahamstown in October 1896 which was published in the South African Medical Journal. His presence at the Institute furthermore allowed Edington to initiate a routine pathological laboratory service, including general examinations of exudations and tissues and public health investigations such as testing the purity of water and milk.
Black played an important role in the development of a controversial method to combat locusts. During the locust plague of 1896 Arnold W. Cooper* recognised a fungus on dead locusts and obtained Black's help to produce a pure culture on an artificial medium. The fungus was produced in quantity and used to combat the locust plague in various regions of southern Africa. During 1897 it was also supplied to East Africa, North Africa, the USA, India, Australia and Argentina, with excellent results. Black published his "Observations on the morphology and conditions of growth of a fungus parasite on locusts in South Africa" in the Transactions of the South African Philosophical Society (Vol. 9, pp. 68-81) in 1896. However, when mycologist I.B. Pole Evans* investigated the matter in 1905 he found that the then available stocks of fungus had no effect on locusts at all and in fact contained a species quite different from that responsible for locust deaths. Furthermore, the deadly species could not be grown in artificial media. He concluded that Black had mistakenly produced a worthless remedy and the technique was abandoned. However, the initial successes with the locust fungus reported from all over the world do not support this conclusion.
Black was the secretary of the Grahamstown and Eastern Province Branch of the British Medical Association in 1896. He joined the South African Philosophical Society in 1897, but remained a member for only a few years. By 1898 he was also a member of the South African Medical Association. About 1897 he left Grahamstown to take up the post of Medical Officer on Robben Island. His work there formed the basis of his dissertation, Leprosy in South Africa, for the degree Doctor of Medicine (MD, University of Aberdeen, 1902). It also led to three papers on leprosy during the next few years: 'Leprosy and its treatment by chaulmoogra oil' (South African Medical Record (Vol. 1, 1903-4), 'Remarks on leprosy in Cape Colony' (The Lancet, 1906, Vol. 167), and 'A new aspect of the pathology and treatment of leprosy' (The Lancet, 1906, Vol. 168).
In October 1903 he was appointed assistant medical officer at the Valkenburg Asylum in Mowbray, Cape Town, where he remained until about 1914. He then obtained a position at the mental hospital in Pietermaritzburg. He was survived by his wife, Blanche I.H. Black, born Wright.