Boston has a history of trading African slaves. There were thousands brought to Early Boston to work in homes and businesses owned by Puritan and Quaker descendants of the Mayflower clans. For it was not long after English Kingdom investors paid the religious freedom-seeking, risk-taking sailors to travel to these shores, when thousands more followed who not as religious, no, they were people interested in capitalism and they had less than perfect characters. These White people began developing the Early Boston colony and they desperately needed slave labor because they were used to working them in the sugar cane fields of Barbados.
Slave cargo was expensive. The average household in Early Boston could only afford one enslaved person. No one had more slaves in the area than the Royalls of Medford. The Royal House Slave Quarters property is there near Tufts University.
This City of Boston webpage is the official position of the City of Boston on its experience with slavery.
Boston City TV official video about its experience with slavery.