https://en.wikipedia.org/wiki/Black_suffrage_in_the_United_S...
https://www.history.com/news/slavery-new-england-rhode-islan...
https://www.smithsonianmag.com/smart-news/why-black-people-a...
And that's just the basic legal stuff, like not being property. When you look at actual attitudes, Northerners were quite racist. You could read, for example, Foner's "The Fiery Trial: Abraham Lincoln and American Slavery". It covers quite well how even Lincoln was quite racist in a way typical of the time.
You might also read Loewen's "Sundown Towns" which shows these attitudes not just persisting, but leading to ethnic cleansing across the US during the Nadir (circa 1870-1920). That includes New England: https://thenewpress.com/books/sundown-towns
There were of course excellent abolitionists and activists from New England, and there were pockets of goodness. But you can't paint New England as a magic exception.