From the limited research I did this afternoon on the topic of slaves living in Union states after 1860, all were in border states of Maryland (87,189), Kentucky (225,483) Delaware ( 1,798) New Jersey (18) Nebraska (15) and Kansas (2). Interesting data, however misleading, and intentionally so.
Kentucky, Delaware and Maryland were Southern states which traditionally practiced slavery but remained loyal to the Union (with some arm twisting). Those in Kansas were brought with slaveholding families from Southern states about 200 at the high point, mostly engaged in farming and domestic work. Slavery ended in Kansas January 29, 1861, except for two who I know they may have been a large threat to the Confederates operating in the Kansas area as those two made it possible for two while men to serve.
Nebraska has a similar situation;
"It is not generally known, but it is a fact, that there were from 1856 to 1858 more slaves in Nebraska than in Kansas. Most of the Kansas slaves were conveyed to the North Star section [the Underground Railroad] soon after. The first attempt to cross the
Missouri River by the new route was made by the Massachusetts party, under the charge of Martyn Stowell, of which I was a member. We were the advance guard in July, 1856, of
Jim Lane's hastily gathered command. The Nebraska City ferry was a flat boat worked by
a Southern settler named Nuckolls, who had brought slaves there and who declared we should not cross. Three of us, who were mounted, rode down, called, and got the ferry over on the Iowa or eastern side of the river with Nuckolls himself in charge, and we held him there until our little company of sixty-five young men, with three wagons, were ferried over. These incidents are only mentioned to show the nature of the obstacles. Mr. Nuckolls yielded to our persuasive force, aided by that of his neighbors, many of whom were free state in sympathy, and perhaps even more by the profit he found by the large ferriage tolls we promptly paid."
I don't know how those few decimated the hopes of the South and doomed it's Antebellum Glory, but I have little doubt I am in for an educating session provided by certain some ones