John Blackett was born on 8 October 1818 in Newcastle upon Tyne, England, the son of John Blackett and his wife, Sarah Codlin. From 1834 to 1840 Blackett was apprenticed as a draughtsman to R. & W. Hawthorn of Newcastle upon Tyne, and worked for this firm from 1840 to 1841. From 1841 to 1844 he was a draughtsman, and from 1844 to 1845 an engineer, with the Great Western Steam Ship Company. He worked under T. R. Guppy from 1845 to 1846, and at the works of the Governor and Company of Copper Miners in England at Cwmavon in South Wales from 1846 to 1848. From 1848 to 1851 he travelled around England and Scotland and was in private practice in Newcastle upon Tyne.
On 19 February 1851 John Blackett married Mary Chrisp at Kirk Leavington, Yorkshire; they were to have four children. Two sons, John George, born on 6 March 1852, and James William, born on 29 October 1855, were to achieve distinction as engineers. After their marriage the couple sailed for New Zealand on the Simlah. Until 1859 they lived in Taranaki, where Blackett farmed at Mangorei. He was also responsible for the construction of several roads and bridges in the province. On 3 June 1858 he was commissioned as an ensign in the New Plymouth Battalion of Militia. In August 1859 the Blacketts moved to Nelson, where John had been appointed provincial engineer. He was responsible for the construction and maintenance of roads, bridges, wharves, the Nelson lighthouse, public buildings and, in 1867, the Nelson city waterworks. In 1861 he joined the Nelson Rifle Volunteers as an ensign.
Blackett's bridge-building won him a silver medal from the commissioners of the New Zealand Exhibition in 1865. He also undertook the exploration necessary to develop the infrastructure needed to exploit the West Coast's gold and coal resources. On 1 August 1865 he was appointed warden of the Nelson South-West Goldfields, and was responsible for the construction of roads from Nelson to the West Coast. He was a member of the Nelson provincial executive in early 1867, but efforts to persuade him to stand for office as superintendent failed.
In 1870, at the inception of Julius Vogel's public works programme, Blackett was appointed acting chief engineer for New Zealand, pending the arrival of an engineer experienced in railway work, and moved to Wellington. He also took responsibility for the colony's marine engineering. In 1871 he became assistant engineer in chief and was formally appointed marine engineer to the general government. In the latter position, which he held until 1889, Blackett made his greatest contribution to New Zealand engineering, with the erection of 14 lighthouses. He wrote a paper on New Zealand lighthouses for the Institution of Civil Engineers, to which he was elected as a member in 1878.
As assistant engineer in chief, Blackett was responsible primarily for New Zealand's roads and bridges. In 1878 he became engineer in charge for the North Island, with more varied responsibilities, including railways. From 1884 to 1889 Blackett was engineer in chief for New Zealand. In 1889, with the winding down of the Public Works Department, he accepted an appointment as consulting engineer for the New Zealand government in England. He resigned in 1892 because of ill health, and returned to Wellington, where he died on 8 January 1893.
Blackett was a hard-working and modest man, who made a substantial contribution to the infrastructures of the province of Nelson and the developing colony of New Zealand.