Blyth was born at 26 Minto St in St Cuthbert's parish, Edinburgh to Robert Brittain Blyth, an iron merchant, and his wife, Barbara, maiden name Cooper. He was their third son, and the first to survive to adulthood.
Blyth was trained as a railway engineer under an apprenticeship with Grainger & Miller, a railway contractor. In 1848 he established an engineering practice on the prestigious George Street (at no 124) in Edinburgh where it would remain for the next 100 years. In 1854, after his brother Edward Lawrence Ireland Blyth finished his own apprenticeship with Grainger & Miller, Benjamin took him into partnership of the renamed B & E Blyth.
Blyth married Mary Dudgeon Wright in Leith, Edinburgh, on 1 August 1848. Mary took on clerical duties in the early stages of Blyth's company. From 1854 they lived at 12 Hope Terrace, Edinburgh. 
His wife died of meningitis in 1868, and their children were subsequently brought up by his wife's sister, Elizabeth Scotland Wright. His eldest son Benjamin Blyth II took over his father's practice and the company remains in business to this day as Blyth and Blyth.
- Dictionary of Scottish Architects entry
- Old Parish Record of birth/christening
- Blyth, E.L.I. 1893, The family of Blythe or Blyth of Norton and Birchet
- Marriage certificate, held by Scottish records office
- Blyth and Blyth: The First 100 Years, historical records held by company
- Death certificate, held by Scottish records office
- Dictionary of Scottish Architects son's entry
- Blyth and Blyth
- Edinburgh Architecture entry
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