He served on the Atlantic Coast early in the war, commanding a brigade in Virginia before being reassigned to administrative duty due to his age and health.

He accepted an appointment to the United States Military Academy in West Point, New York, and graduated 26th in the Class of 1829.

Initially given the rank of brevet second lieutenant in the 3rd U.S. Infantry, he was assigned to various bases on the Western frontier duty, as well as performing recruiting services and helping engineer improvement to the Sabine River.

Blanchard returned to the army in May 1846 with the outbreak of the Mexican War, serving as a captain of the volunteer "Phoenix Company" in the 2nd Louisiana Infantry.

Serving with distinction in the Battle of Monterey and the Siege of Vera Cruz, he was commissioned as a major in the Regular Army in the 12th U.S. Infantry.

When the Civil War erupted in 1861, Blanchard offered his services to his adopted state and received a commission as the colonel of the 1st Louisiana Infantry regiment.

Afterward, he was replaced by Ambrose R. Wright because of his advancing age and the desire for a younger officer to lead the brigade in the field.