author's name, ‘An Enquiry into the Life and Writings of Homer,’ arranged in twelve sections, as an answer to the question, ‘By what fate or disposition of things it has happened that no poet has equalled him for 2,700 years, nor any that we know ever surpassed him before?’ A second London edition in octavo, and also anonymous, came out in 1736, followed soon after by ‘Proofs of the Enquiry into Homer's Life and Writings, translated into English; being a Key to the Enquiry ....’ With a curious Frontispiece, 8vo, London, 1747. This was merely a translation of the learned and copious notes originally given in Greek, Latin, Spanish, Italian, and French. The ‘Enquiry’ was considered a remarkable book at the time, and opinions on its merits have varied considerably. Gibbon, without any explanation of his assertion, speaks of it as ‘by Blackwell of Aberdeen, or rather by Bishop Berkeley, a fine, though sometimes fanciful, effort of genius!’
In 1748 appeared another work by Blackwell, ‘Letters concerning Mythology,’ 8vo, London, without his name or the bookseller's (Andrew Millar) imprint. The preface intimates that some of the first letters ‘passed in correspondence written by a learned and worthy man, whose death prevented his prosecuting his plan,’ the additions to the seventh and eighth letters, and all following, being by the author of ‘An Enquiry .... Homer,’ &c. No clue is afforded to the original writer, whose letters are given in a very pleasant and lively style, and chiefly refer to the Homeric ‘Enquiry.’ The later writer continues throughout in the same vein, and makes a very readable book. The second edition, 8vo, London, 1757, appeared soon after the author's death, and gives his name. In the first volume of the ‘Archæologia’ there is a letter, dated 18 Aug. 1748, addressed by Dr. T. Blackwell to Mr. Ames, with an explanation of an ancient Greek inscription on a white marble found in the Isle of Tasso by Captain Hales.
On 7 Oct. 1748 George II appointed Blackwell principal of the Marischal College in Aberdeen, a position which he held, along with the Greek chair, till his death. Blackwell is the only layman ever appointed principal of this college since the patronage was vested in the crown. When the well-known Glasgow printers, Robert and Andrew Foulis, projected an edition of Plato, Blackwell proposed to furnish them with critical notes, together with an account of Plato's life and philosophy; his terms being too high, the design was relinquished. He then published in the ‘Gentleman's Magazine’ for 1751 a Latin advertisement of a similar venture of his own. This work was never published, however, and his manuscripts, after death, offered no traces of such a scheme.
On 30 March 1752 he took the degree of doctor of laws, and in the following year appeared the first volume of his ‘Memoirs of the Court of Augustus,’ 4to, Edinburgh. The second volume was published, 4to, Edinburgh, in 1755, and the third volume, which was posthumous and left incomplete by the author (whose text reached to p. 144 only), was prepared for the press, with additional pages, by Mr. John Mills, and published in 4to, London, 1764 (seven years after his death), along with the third edition of the two former volumes. This work contains fine impressions of heads of great personages from genuine antiques. It had a good reception, but unfortunately it was written with so much parade and in such a peculiar style that it offered a wide field for adverse criticism. Johnson reviewed it sarcastically in the ‘Literary Magazine,’ 1756, but concludes: ‘This book is the work of a man of letters; it is full of events displayed with accuracy and related with vivacity.’ A French translation by M. Feutry of this work was published in 12mo, 3 vols., Paris, 1781.
Several years before his death Blackwell's health began to decline, and compelled him to take assistance in his Greek class. Eventually he was forced to travel, and in February 1757 he reached Edinburgh, but could proceed no further. In that city he died on 8 March, in his fifty-sixth year. During a protracted illness he had displayed an equable flow of temper, endearing him to all. Before he started on his journey he drew together all the professors of the college and spent two hours of pleasant conference with them, and on the day of his death he wrote letters to several of his friends, and took leave of them in a cheerful and contented strain. In private life his habits were very agreeable; his conversation ever instructive and affable, accompanied with a flow of good humour, even when provoked to some display of passion.
Soon after his appointment as principal of his college he married Barbara Black, daughter of an Aberdeen merchant, by whom he had no children. This lady survived him many years and died in 1793. She bequeathed her estates, partly to found a chair of chemistry in the college with which the names of her husband, her father-in-law, and the Fordyces (her nephews) had been so long associated, and partly for the premium of an English essay and for the augmentation of the professorial salaries.