William Newsham Blair was born at Kilmeny, Islay, an island in the Scottish inner Hebrides, probably on 10 August 1841. He was the son of Mary Newsham and her husband, David Blair, a substantial farmer and miller. After a good parish school education Blair was articled to a civil engineer and surveyor in Oban. At the age of about 20 he went to Edinburgh as assistant to a well-known civil engineer, but was laid off and returned to Islay. Unable to find work, he sailed to Port Chalmers, New Zealand, on the Daniel Rankin, arriving in December 1863.
The opening up of the Otago hinterland had necessitated the development of an engineering infrastructure, and Blair was immediately employed by Otago's chief engineer of roads. An early task saw him surveying roads near Queenstown, in winter and under canvas. Later he was involved in constructing wharves and waterworks at Port Chalmers, and surveying and planning many of the Otago and Southland railways. On 22 January 1867, in Dunedin, Blair married Mary Kennedy, to whom he had been engaged before emigrating.
Blair was appointed chief engineer of railways when his predecessor drowned in 1869, and district engineer in the general government's new Public Works Department shortly after it took over the country's railways. Until 1878 he was responsible for the construction of all Otago and Southland's railways, and many of their roads and bridges. In 1872 he constructed the long Rangitata Bridge in Canterbury. He was consultant to the Dunedin City Council for various matters, including, between 1876 and 1881, the Silverstream Waterworks – the city's mainstay for 75 years.
In spite of being exceptionally busy with railway construction Blair found time to be active in community affairs. He was convener of the building committee for the new Knox Church; presented papers to local scientific societies; researched the engineering properties and economic worth of local building materials, and in 1879 wrote a standard reference book on them; and gave public addresses on Scottish literature and poetry (which he had studied in Scotland). He became a member of the Institution of Civil Engineers, London, and became an authorised surveyor in 1877.
In 1878 the government divided the Public Works Department in two and Blair was put in charge of the South Island division. His 1879 exploration with C. Y. O'Connor of railway routes across the Southern Alps in Canterbury and Nelson resulted in a comprehensive report in 1884. Changing government policy re-amalgamated the department in 1884, and Blair was ordered to Wellington as assistant engineer-in-chief. He and his family shifted there in mid 1884.
Just before leaving Dunedin Blair gave a remarkably perceptive address to the inaugural meeting of the New Zealand Manufacturers' Association; he repeated it by invitation three years later in Christchurch. He advocated with masterly argument and detailed supporting evidence the value to the national economy of import substitution, tourism, and even the hydroelectric energy potential of the country's rivers. Later he wrote articles on the southern lakes, and one on the adverse effects of mining and deforestation on the landscape and coasts. The latter article was an example of his foresight, keen observation during explorations, and intimate knowledge of New Zealand's natural environment.
While assistant engineer-in-chief Blair carried through the construction of the Otago Central Railway, which he had earlier surveyed. This involved heavy constructions such as the Wingatui viaduct and other much-admired structures. He also explored the King Country to report on the proposed North Island main trunk railway.
In May 1890 Blair became engineer-in-chief and under-secretary of the Public Works Department, thus reaching the pinnacle of the engineering profession. His health failed, however, and he died in Wellington on 4 May 1891 after a lengthy illness. His body was taken to Dunedin for burial from Knox Church. His early death is thought to have been hastened by the struggle for promotion against the claims of his former subordinate, O'Connor. He was survived by his wife, four sons and two daughters.
William Blair was loved and respected by his staff, and in private life was the centre of a large circle of friends. Senior government ministers were among the pall-bearers at his funeral, and the Dunedin newspapers reported the progress of his cortège from Wellington to Dunedin. He was remembered both for his achievements in engineering and for his capacity to inspire lasting affection. 'Simple and modest ever, he…loved a word of friendship better than a column of praise.'