The reaction of free blacks to slave codes largely depended on where they lived. According to the Library of Congress, many free blacks in the South could do little in the way of opposing the slave codes because they were barred from travelling or assembling peacefully. In Northern cities, free blacks opposed the slave codes through voting, writings and buying slaves who were friends or family members.
Free slaves in the North would coordinate with white abolitionists to form escape routes for runaway slaves, according to the Library of Congress. Biography.com mentions Frederick Douglass as one of the most prominent former slaves who became an ardent abolitionist. Other freemen like Benjamin Banneker and Phillis Wheatley were also critics of the slave codes in their writings.
There were other free blacks who were not so fortunate, and the Washington Post notes the story of Solomon Northup, a freeman who was captured as a slave for twelve years. He was subsequently freed by friends and later became a popular voice within abolitionist circles by sharing his story and participating in the Underground Railroad.
Other free blacks responded by having documentation at all times to keep themselves from being captured as slaves. Ancenstry.com notes that freemen were required to register their free status within local counties. Without such paperwork, they stood the risk of being sold into slavery, even within the Northern states.