There are no Lancashire deaths indexed (from Sep 1837) for a William Blackley born before 1800. He married in 1803, so must be much older!
He most likely died in 1820, aged 46 (confirmed by Probate).
Name: William Blackley
Age: 46
Birth Date: abt 1774
Burial Date: 13 Jul 1820
Burial Place: Bolton-le-Moors, St Peter, Lancashire, England
Reference Number: GB127.L243/1/5/3
Name: Bleakley, William
Dates: 21 Sep 1820
Place: Great Bolton, Cheshire, England
Collection: Cheshire: - Wills and Inventories, 1811-1820 (A-L)
Volume: List of Wills and Administrations (Infra Wills are marked with an asterisk) Now Preserved in the Probate Registry at Chester, For the Years 1811-20, both Inclusive.
Chapter: [1700]
Text: Bleakley, William, of Great Bolton, manufacturer. 21 Sep 1820.