After graduating from Princeton University, Blackford moved to the Indiana Territory to practice law in 1812. After holding several civil service positions, he was appointed a circuit court judge but resigned just before the territorial government was dissolved in 1816. Elected as a representative to the first session of the Indiana General Assembly, he was chosen to serve as the first Speaker of the Indiana House of Representatives. Following the death of Indiana Chief Justice John Johnson in 1817, Blackford was appointed as his replacement by Governor of Indiana Jonathan Jennings. Blackford's early important cases included "Polly v. Lasselle", the decision in that case freed all slaves in Indiana. Nominated without his knowledge or permission, in 1825 he was the Whig candidate for Governor of Indiana, but was defeated in the election because of his refusal to campaign publicly. Again nominated without his knowledge to become a United States Senator, he lost the election in the Indiana General Assembly by only one vote.