What does the blue mean on the 1860 map

The debate in the Upper South (light gray) lasted longer, but by the middle of 1861, they too, seceded. Lincoln knew that the Border States (light blue), where slavery was permitted, were crucial to the Union cause. The keep them in the Union (dark blue), he initially refused to free slaves as part of the war effort.

Explanation:

what are the choices btw if it was me id say the lack of money to continue with education