[asheshouse]

The black areas will always appear if the area is designated as land on map_c but does not have a land texture allocated on map_T. Because of the difference in pixel scale between map_c and Map_T it is quite common to have some parts of the coast which do not line up quite right on the first attempt.

For a quick fix, working in FMB Map Editor mode go to Objects/Tile/Lowland0 and as you apply the texture you will see the area colouring in.