A London banker, born in Tewkesbury, he joined Martin's Bank in 1731 and later became a partner. He was a close friend and correspondent of the Wesleys for over 40 years. John Wesley confided in him on both financial and marital problems and repeatedly exhorted him to avoid worldliness, writing in 1753, 'I feared you was not so bold for God as you was five years ago.' Blackwell was somewhat addicted to gambling. But he gave much financial support to Wesley's philanthropic activities and was one of the first trustees of Wesley's Chapel. The Limes, his country home at Lewisham, was one of John Wesley's favourite retreats, where a number of his sermons and other writings were prepared and where, on 24 November 1777, he had a cordial meeting with Robert Lowth, Bishop of London and uncle to Blackwell's second wife. He died on 21 April 1782 and Charles Wesley wrote memorial verses on him as one who had 'spent his life in doing good'.
'Blackwell became a close friend of John Wesley, but he was a man of the world, and did not quite live up to Wesley's ideals. Writing in 1753, Wesley complained: "I feared you was not so bold for God as you was five years ago." ...
'Blackwell fancied himself, nevertheless, as a Chrtistian mediator, offering to resolve the friction between John Wesley and his brother Charles, to whom he wrote as follows: "It is not my present design to enter into any arguing about the cause of the unhappy difference which now subsists between you and your wife and Mr. John Wesley and his wife, but this from my soul I say that if you and Mrs. Wesley will but come to town and spend one week or fortnight here, I am persuaded with the assistance of a friend or two we shall be able under the discretions of our God to put all things in such a situation as will make you all perfectly easy and perfectly happy."
'Blackwell helped John Wesley financially. In 1748 he gave £20 towards an Orphan House; in 1753, 10 guineas towards a tabernacle. He practised Christian charitry in other directions
'Blackwell was too fond of gambling for a Christian banker. He would lay bets on any excuse. He bet on reduction of the Land Tax, on the date of the termination of Parliament, and on the matrimonial chances of his friends...
'Blackwell lived in considerable style. In his accounts 1750-54 he recorded the purchase of claret, cider, green tea and chocolate. He lived as befits a man who, as Wesley wrote: "had enough, and that, by the blessing of God, he knew it." '
George Chandler, Four Centuries of Banking, Vol. 1 (1964) pp.129-30