Ok, here's what I've further discovered.
If I insert a floor without walls, apply grass to the underside of the floor and then lattice to the edge of the floor, it's perfect for the look I want. Do not have to do even one other change. Sits perfectfully on the landscape, no blue line. But as soon as I add walls, don't drop the exterior siding and apply the exact same effects to the floor, same elevation, everything is the same except have added the wall, I get the blue under the floor in spite of what I've tried.
Is there something else I can do or is this just a glich in relation to walls?
Yellowcense