It's because there are no objects on the corner of your tile. It's happened to me before, my suggestion is you put a small structure, such as one of those small houses that's hard to see. I used it as pylons in my racing map for Jeeps.
I think it's named str7, not sure. But I do know that it's turning blue because there are no structures on the outside edge of the tile (Particulary the corners. NE, SE, SW, NW.)