in or characteristic of a region of the United States north of (approximately) the Mason-Dixon line
is similar to
used to signify the Union forces in the American Civil War (who wore blue uniforms)
"a ragged blue line"
is similar to
being of or having to do with the northern United States and those loyal to the Union during the American Civil War
"Union soldiers"; "Federal forces"; "a Federal infantryman"
is similar to
used by Southerners for an inhabitant of a northern state in the United States (especially a Union soldier)
is an attribute of
the United States (especially the northern states during the American Civil War)
"he has visited every state in the Union"; "Lee hoped to detach Maryland from the Union"; "the North's superior resources turned the scale"
is an attribute of
the region of the United States lying to the north of the Mason-Dixon line