Blackwood, Sir John (1722–99), 2nd baronet and MP, was eldest son among three children of Sir Robert Blackwood (1694–1774), landowner and 1st baronet, of Ballyleidy, Upper Clandeboye, Co. Down, and his first wife Joyce, sister of Joseph Leeson (qv), 1st earl of Milltown. He matriculated at Glasgow University (1739) and graduated MA (1741). At Glasgow, according to Professor Francis Hutcheson (qv), Blackwood spent too much time in taverns and encouraged his fellow students to do likewise, but otherwise was of good character. He was MP for the family borough of Killyleagh, Co. Down (1761–8, 1776–90, 1797–9) and for Bangor, Co. Down (1768–76, 1790–97), although he did not attend parliament regularly. His eldest son, Robert Blackwood (1752–86), was MP for Killyleagh (1776–86), and like his father usually opposed the government, at least until the viceroyalty of Portland (qv) in 1782; a government observer noted that Sir John was a man of ‘large property and presbyterian family’ (Sayles, 244). A member of the Whig Club of Co. Down, he generally supported catholic emancipation and parliamentary reform, voting for the major catholic relief bill of 1793. In the bitter Co. Down electoral battles of the 1790s between the Stewart and Hill families, he strongly supported his east Down neighbours the Stewarts; he was also connected politically with Francis Seymour-Conway (qv), 1st marquess of Hertford, who as viceroy of Ireland (1765–6) made his father a baronet in 1763, and with Bernard Ward (1719–81), 1st Viscount Bangor, who secured his election for Bangor in 1768. Criticised by more zealous magistrates for his failure to suppress the United Irishmen, he fled to Belfast during the 1798 insurrection until peace in his locality was restored. He greatly valued his political independence and was offended by Dublin Castle's summary request for his support for the union. In fact, he strongly opposed the union and resisted offers of a peerage to change his mind. He was preparing to go to Dublin to vote against the union when he died at Ballyleidy on 27 February 1799; it was said that he collapsed in the act of putting on his boots for the journey. His eldest son Robert having been killed by a fall from his horse on 4 February 1786, Blackwood was succeeded in the baronetcy by his second son, James Blackwood (qv), who voted for the union.
John married (22 May 1751) Dorcas Stevenson (d. 1808; cr. Baroness Dufferin and Claneboye 1800), eldest daughter and co-heir of James Stevenson of Killyleagh, and granddaughter of James Hamilton, heir general of the earls of Clanbrassill, who brought into the family possession half of the town of Killyleagh and an estate in the barony of Dufferin. They had seven sons and four daughters.