"Pockets of slavery existed in the North up until the end of the war, and Yankee slave ships continued to ply their trade during that time."https://slavenorth.com/newjersey.htm
Certainly, the border states like Kentucky, Missouri, etc. had slaves, but what non-border union states had slaves? And what northern ships were plying the slave trade many decades after it, slave ships, had been abolished?
All irrelevant anyway.
If the north had been hypocrites about slavery, they would not have passed the 13th amendment abolishing slavery.
...In 1830, of the 3,568 Northern blacks who remained slaves, more than two-thirds were in New Jersey. The institution was rapidly declining in the 1830s, but not until 1846 was slavery permanently abolished. At the start of the Civil War, New Jersey citizens owned 18 "apprentices for life" (the federal census listed them as "slaves") -- legal slaves by any name.
"New Jersey's emancipation law carefully protected existing property rights. No one lost a single slave, and the right to the services of young Negroes was fully protected. Moreover, the courts ruled that the right was a 'species of property,' transferable 'from one citizen to another like other personal property.' "
Thus "New Jersey retained slaveholding without technically remaining a slave state."...