Blacket was born at St Margaret's Hill, Southwark, Surrey, England on 25 August 1817. His father was the merchant James Blacket and his mother was Margaret Harriet (nee Ralph). He was educated at Mill Hill Congregational College and then spent three years in the family trade business before deciding to pursue a different calling. Initially he found employment in his brother's mill in Yorkshire before joining the Stockton and Darlington Railway Company around 1837 as a surveyor and engineer. While working for this company he became a skilled draftsman and surveyor, which were skills that were to serve him admirably later in life. His success as an engineer prompted his father to reward him with the funds to undertake a year travelling England sketching and recording details of medieval architecture. Between 1838 and 1841 he undertook many similar smaller journeys during which he produced many picturesque sketches of medieval architecture. After leaving Yorkshire in 1841 Blacket spent a year in London employed in inspecting schools for the Archbishop of Canterbury.
