I don't know how to use curved, dark map paths, but I did put straight, dark paths on the overworld map. When I edit this on Lunar Magic, these layer 2 maps only have light paths. I have several paths that are not connected because of gaps involving missing curved, dark lines.
How to use a curved, dark map path?
With the 8x8 editor, you can create and rotate everything you need. If you want them to be layer 2 tiles specifically, you simply place your creations next to the already existing ones.