Blackford was enrolled in Princeton University at age sixteen and graduated at the top of his class in 1806. On his enrollment forms he entered his middle name as Newton, but never again used the name during his life. Among his fifty-four classmates were John Williams Walker, James Iradell, John James Marshall, and Patrick Noble. Blackford became fluent in Greek and Latin and excelled in history. He also spent a considerable amount of time reading legal books on English common law. After college he studied law for a year in the office of Revolutionary War hero Colonel George MacDonald before moving on in 1810 to complete his legal studies in the office of New Jersey Supreme Court Justice Gabriel Ford in Morristown. He was admitted to the bar in New Jersey that same year. He took only one case of his own and realized he needed to join a law firm if he was going to be able to earn a living at his profession. He returned to his hometown and joined MacDonald's firm.