Blackwell was employed as an engineer on the Kennet and Avon Canal in 1806, working primarily as site agent on the Caen Hill Flight in Devizes, Wiltshire.

[2][3] While John Rennie designed the flight,[a] it was likely that construction was undertaken by the Kennet and Avon Canal Company and its engineers.

[2] In 1808, shortly before the completion of the flight, Blackwell moved to work with Rennie on Crofton Pumping Station.

[5] Blackwell was appointed the company's resident engineer on 19 July 1814, with a salary of £300 (plus £50 expenses to cover a horse).

[2][3] Blackwell subsequently undertook essential maintenance of the waterway, ensuring passage for fast boats could be made—allowing transport between London and Bristol in five days.

[3] In his research, Blackwell travelled to the north of England to visit some of the early railway systems, but remarked that "no great improvements have been made, there are limits to their powers which are nearly approached".

[3] In 1832, Blackwell met English engineer Isambard Kingdom Brunel, who described him as a "bigoted, obstinate, practical man.

THEY DIED AT HUNGERFORD, AND WERE INTERRED IN A VAULT IN THE ADJACENT CHURCHYARD[1]A plaque dedicated to Blackwell was erected on Prison Bridge, Devizes, after his death, reading: "THIS TABLET IS ERECTED BY THE KENNET AND AVON CANAL COMPANY TO THE MEMORY OF JOHN BLACKWELL, WHO DURING THIRTY-FOUR YEARS SUPERINTENDED THE WORKS OF THE CANAL AS THEIR ENGINEER WITH FIDELITY, VIGILANCE, AND ABILITY"Thomas became a civil engineer, and upon his father's death became engineer of the Kennet and Avon Canal.