I think it is ludicrous to suggest -- as so many do -- that the North cared at all about blacks in any meaningful way. Lacking any large economic incentive for keeping slaves, the few abolitionists had it relatively easy getting such laws passed. Some individual states had outlawed slavery -- most had by the start of the war -- but "freedom" was not for black people.

Slaves were not freed in the border states (Union) until the passing of the 13th Amendment in 1865. (KY, DE, MD, MO). The infamous 1857 Dred Scott decision is the most well known, but it was only one in a series of Supreme Court Cases where blacks were determined not to be citizens, and which upheld their slave status even in supposedly "free" states.

In 1861, Union Gen. Butler forced fugitive slaves to labor for his army. So much for "freedom," right. Slavery was not outlawed in DC until April 1862, and was not outlawed in the territories until a couple of months later. Lincoln repeatedly repealed his generals' edicts to free slaves in certain areas for supposedly Constitutional reasons, yet had no problem later issuing an equally Un-Constitutional Emancipation Proclamation.

No, slavery was not illegal in the Union states as a whole. Abolition was not treated as a Federal issue until after the start of the war.