The blue line is arbitrary. Its simply a graphical representation that two items are aligned. It doesn't matter where its located in two views that are oriented the same way. More importantly, there could be multiple blue lines and when one is aligned, they are all aligned. That is the distinction here. If you have two elevations looking from opposite directions, and you have two arbitrarily placed planes, you can never align them both. Only one.
So when you try to align opposite elevations/sections, Revit has no idea which line to align to. Even a "mid-line" is arbitrary. Mid-line of what? Does it include the topography? Just the building? What if there are multiple phases? And you may think you have answers to all of that, but how does Revit know? And then why is that information even relevant?