As you can see in the photos, when "Out the map" blocks are in a dark location, in this example under the water, they become very dark to the point where its very hard to differentiate between the different blocks. In addition to this, when a "Out the map" block is painted a glowing colour, it doesn't light up said block, nor blocks around it. This means that lighting up dark areas "Out the map" is bascially impossible.
My suggestion is to make glowing blocks outside the map work the same as they do inside the map, so lighting up an area around the painted block. This would allow players to have more freedom when building out the map, and allow them to control the lighting for darker areas