Governor Edward Coles appointed Blackwell Illinois Secretary of State on April 2, 1823. Blackwell's goal in the position was to prevent a constitutional convention in which pro-slavery forces could reverse Illinois' de jure status as a free state. This goal, achieved in part by Blackwell's control of the Illinois Intelligencer, was accomplished when the referendum failed. Blackwell resigned as Secretary of State on October 15, 1824.[1]
After his time as Secretary of State, Blackwell returned to the Illinois House of Representatives for the 4th General Assembly which commenced November 15, 1824.[2] On January 2, 1826, he was elected Speaker of the Illinois House of Representatives to replace Thomas Mather who resigned to take a job to locate a military road.[1] He was reelected to the 5th General Assembly which convened December 4, 1826. The session adjourned February 19, 1827, and the assembly ended December 1, 1828.[2]
Blackwell died on March 28, 1832, in Belleville, Illinois.[1]