Mobile ADF raster implementation support supports layers, but not color key transparency. As a result, two layers can be blended by making one of them semi-transparent. It is not possible to assign transparency to one value; for example, ‘no data’.
To avoid seeing black artifacts in areas where an image has no data, use raster that has data throughout the extent. Also, turn layers on and off depending on whether it obscures visibility and the use of other layers.