The total area that your cursor can access in the Town Map is 22 x 14 8x8 tiles. Basically this area is 176 x 112 pixels in size.
The blue thing should be black. How do I solve thís?
That's because you're using the wrong type of block when making that tile. In FR's tileset 0 the top left tile looks to be entirely black, when in fact it is not. This is because the black that makes up the tile is being used as Palette 0's transparent colour. As such, when you apply nothing but that transparent 8x8 block when making the tile you are seeing 'beyond the tile'. If you wish to have a completely black tile that shows up in tileset 0, you are going to have to change one of the palettes associate with the tileset so that it has a (non-transparent) colour that is perfectly black.
You can see why the tile actually comes out as being black when playing in interior maps because tileset 12's palette 5 has a colour that is both black and not transparent which is used to make up the tile.