Black tiles are a known glitch in AWBW. They are zero movement cost terrain, but cannot have a unit end its movement on them. When there are black tiles in the upper left corner of the map (coordinates 0,0) then movement over any black tiles connected to that one becomes glitched. It is a terrain only available through uploading maps onto the site.
Black tiles have become commonplace enough to where it is likely a bug that won't be fixed. Several maps feature them, including some fun gimmick maps. Maps with black tiles are generally not let into leagues or tournaments because of the potential bugs that may come with them.