In 1821, Representative-elect John S. Richards (DR), who'd been elected to represent South Carolina's 9th district, declined to serve. A special election was held to fill the resulting, the first of two special elections in the 9th district for the 17th Congress.
Election results
Blair took his seat at the start of the 17th Congress. Blair himself subsequently resigned May 8, 1822, resulting in a second special election.
References
South Carolina's 9th congressional district special election, 1821 Wikipedia(Text) CC BY-SA