After preparatory studies at Chester Academy in Chester, Vermont, Blanchard matriculated at Middlebury College in 1828. Upon graduation in 1831, he was named preceptor of Plattsburg Academy in Plattsburgh, New York. He taught for two years but found the work dissatisfying. Blanchard first supported abolitionism in 1834, believing slavery to be inconsistent with Biblical teachings. He then enrolled at the Andover Theological Seminary in Andover, Massachusetts. While there, the school denounced the American Anti-Slavery Society (AASS) and demanded that students reject abolitionist views. Blanchard left the school in 1836 to join the society, which assigned him to preach in southern Pennsylvania as one of Theodore Dwight Weld's "seventy". Blanchard was stoned in the streets by citizens in Harrisburg, Pennsylvania in 1837. Nonetheless, he was considered one of the most effective agents of the AASS and is credited with converting Thaddeus Stevens to the abolitionist cause.