On the black background, the left side appears lighter because it contains 50 % of black, so normally it looks lighter than 100% black (the background).
On the blue background, I haven`t changed anything (except the background - from black to blue), The blue is 25% cyan so the 50% percent black from the left looks (normally) darker than the blue because it floats over it.
Here is a link where I put 4 different background under the map. As you will see, it works automatically on all of them.
http://tinypic.com/r/4heecp/5