Eauxgod! You just make it up as you go along. The US abolished the importation of slaves in 1808. There was some small clandestine and illegal trade going on afterwards to Texas, Florida and South Carolina, but do you really think a slave ship could have sailed into Boston harbor illegally during the Civil War with slaves on board without attracting notice and condemnation? And what exactly would have been done with those slaves? If you have some real evidence that slaves entered the port of Boston as late as 1863 present it now, or stop making such claims. More information
here.