Henry Blackett was born in Durham, England, probably in 1819 or 1820, the son of Margaret Bell and her husband, Cuthbert Blackett, a worsted and carpet manufacturer. He was educated at the local grammar school and later opened a woollen drapery firm. Blackett married Isabella Benton in Longframlington, Northumberland, on 2 December 1850, and in 1858 on account of ill-health brought his family to New Zealand. He seems to have intended to settle in Taranaki, but on reaching Nelson heard of Taranaki's unsettled land problem. He decided that there were few opportunities for trade there and the family moved on, reaching Lyttelton on the schooner Gypsy on 30 May 1858.
Blackett bought land in Rangiora, 20 miles north of Christchurch. He built and leased shops, and established a large store which sold hardware, groceries, and wines and spirits, the latter imported direct from England. The spacious cellar of his shop became a place for the civic-minded of Rangiora to debate the issues of local politics.
Blackett soon became a member and chairman of the Mandeville and Rangiora Road Board. In the 1860s, as a member of a drainage board set up under the auspices of local road boards, he was the author of a plan for draining the great swamp south of Rangiora to open up land for farming: a main drain with subsidiary drains arranged in herring-bone fashion would take away stagnant water and seasonal flood waters.
In the early 1870s the Canterbury provincial government planned to have the Christchurch–Hurunui railway line pass between Rangiora and Woodend. Each town, eager to become the area's principal market town, fought for the right to host the railway; neighbouring smaller towns took sides. Blackett was a persuasive advocate for Rangiora, successfully enlisting the support of William Reeves, then resident minister for the Middle Island (South Island). His triumph came on 5 November 1872; with a holiday crowd at his back, he welcomed the superintendent of Canterbury on the first of the northern trains.
Similarly, the district of Eyre competed with Rangiora for the right to host the branch line to Oxford. Blackett adopted more strenuous tactics in this case. On one occasion, a public meeting at West Eyreton was abandoned when news came that Blackett and a host of Rangiora men were about to arrive. Blackett re-opened the hall and held his own meeting; the following morning's papers announced that a meeting at West Eyreton had supported the Rangiora route. Both branch lines were eventually constructed.
In 1876 provincial governments were abolished in favour of a multitude of counties. As a member of the first Ashley County Council, Blackett was one of the few to recognise that the new body could do more for the area's development than the old road boards with their limited powers and finance. A more blinkered view prevailed, and the county had little success. Blackett also fought strenuously for the incorporation of Rangiora as a borough; as its first mayor from 1878 to 1880 he insisted on sound financial organisation taking precedence over grandiose schemes for development. He served a second term as mayor from 1887 to 1888.
Isabella Blackett, who had of necessity played a major role in the management of the family business, died at Rangiora on 19 August 1906. Henry Blackett died there on 11 July 1907, survived by five daughters and four sons. Pugnacious by nature and intensely parochial, he had played a major role in the development of North Canterbury, and is commemorated, along with his wife, by an illuminated tablet in the Anglican Church of St John Baptist, Rangiora.