Daniel is correct. The "blackboard" used in mobile homes is used purely for economy. Additionally, mobile homes are not required to to have structural sheating as in OSB or plywood in the corners. They typically handle the shear component by using metal strapping, sometimes a let-in brace and occasionally a shear panel. One thing to keep in mind is that mobile homes are not built to standard building codes (BOCA or UBC, etc) but are built to HUD codes specifically for mobile homes.