Blackney came from an old Carlow Catholic family, his great-great-grandfather and namesake having inherited Ballycormack in the 1680s from a maternal grandfather, Dudley Bagenal, the son of George Bagenal of Dunleckny, who sat for the county in the Irish Parliament of 1613. On the death of his father, 28 Mar. 1796, Blackney became heir apparent to the family estates of his grandfather Walter, who had acquired additional leasehold properties at Ballyellin and Clonmoney in 1781, to which Blackney succeeded, 29 Sept. 1796.
In the House he was ‘popularly called Dr. Doyle’s Member’, on account of the bishop’s part in his campaign.
Blackney voted for the Grey ministry’s reintroduced reform bill, 6 July, and gave generally steady support to its details, though he voted against the division of counties, 11 Aug. 1831. He divided for the bill’s passage, 21 Sept., the second reading of the Scottish bill, 23 Sept., and Lord Ebrington’s confidence motion, 10 Oct., and congratulated ministers on their ‘great measure’ the following day. He paired for the second reading of the revised bill, 17 Dec. 1831, and again supported its details, though he voted against the enfranchisement of £50 tenants-at-will, 1 Feb. 1832. On 11 Feb. he defended the use of election oaths against Croker, whom he accused of speaking ‘only for the purpose of delaying the bill’. He was absent from the divisions on the third reading, 22 Mar., Ebrington’s motion for an address calling on the king to appoint only ministers who would carry the bill unimpaired, 10 May, and the second reading of the Irish bill, 25 May. On 6 June he denounced the ‘extensive injustice’ of the Irish registration system, describing how many of his tenants had been ‘refused registration because they were not able to produce my title as well as their own’ while he was attending the House. He presented and endorsed a petition against the ‘present abominable system’ from Borris, where he claimed that only three out of 150 had recently been allowed to register, and warned that ‘if permitted to continue’ this would ‘render the measure of reform abortive’, 14 June. He divided for O’Connell’s motion to extend the Irish county franchise to £5 freeholders and was in the minority of nine to enfranchise £30 rent payers on leases of 19 years, 18 June. He voted against the liability of Irish electors to pay municipal taxes before they could vote, 29 June, when he complained that ‘no one appears to know what a register is, what a freehold is, or what anything is’. He called for a jury system to replace the predominantly Protestant Irish revising barristers, with whom the people were ‘by no means’ satisfied, 2 July, and demanded the removal of county Carlow’s assistant barrister who was hostile to the ‘rights of the people’, 6 July 1832.
Blackney was in the minority for the Vestry Act amendment bill, 23 Jan. 1832, when he asserted that Ireland was determined ‘no longer to submit to the payment of tithes’. He protested that the exclusion of Catholics from the tithes committee would ‘not reconcile a pauperized population’ to their payment, 9 Feb., and that it was ‘not in the power of England to maintain the present system’, 14 Feb. He rejected as a ‘foul stigma’ allegations that he had gone from ‘chapel to chapel’ through Carlow with ‘a mob at his heels’ proclaiming ‘no tithes’ and ‘no Union’, 16 Feb., when he spoke and voted for printing the Woollen Grange petition for the abolition of tithes. He divided steadily against the Irish tithes bill thereafter, blaming the clergy for creating the problem, 2 Mar., and asking how long ministers intended to ‘persevere in their wild project of coercion’, 13 Mar. On 14 June he declared that without the ‘speedy extinction’ of tithes there would be an ‘extinction of lives, and a levelling of all property’, but he was called to order by the Speaker for repeating the observation of a clergyman opposed to the reform bill, who ‘did not scruple to tell me that the present was not the first king who had brought himself, by his acts, to the block’. He blamed the ‘misery’ of Ireland on ‘corruption, misgovernment and absenteeism’ and voted for a tax on absentee landlords to provide permanent provision for the poor, 19 June. He contended that ‘quite enough’ had been done to relieve the Irish clergy and ‘rich prelates’, 29 June, and, in a speech for which he was again reprimanded by the Speaker, charged them with ‘oppressing the poor of Ireland’, 3 July. He warned ministers that any attempt to protect tithes would prove ‘utterly impossible’, even if they tried to use ‘the whole standing army of England’, 10 July. He divided for inquiry into the glove trade, 31 Jan. On 13 Feb. he condemned remarks made by Thomas Lefroy against the Catholic religion, which had withstood ‘the attack of fire and sword’ and ‘persecution and slaughter’ and would not ‘be overthrown by such puny assaults’. He spoke against any alteration to the Irish tobacco growing regulations that would ‘be advantageous to tobacconists’ but not ‘to the country at large’, 29 Feb. He voted for inquiry into the Peterloo massacre, 15 Mar., and against the government’s temporizing amendment on the abolition of slavery, 24 May. He voted against Baring’s bill to exclude insolvent debtors from Parliament, 6 June. He voted for a system of representation for New South Wales, 28 June, and military reductions, 2 July 1832.
At the 1832 general election Blackney was returned for county Carlow as a Repealer after a contest. He retired at the 1834 dissolution to make way for Maurice O’Connell, whom he duly proposed.
