I'm not sure, but I think the dark edges are part of the night lighting and not the map itself, and the only answer is to move the map so the part you want to see is in the middle.
You can check out these and see if one of them helps.
http://www.subsim.com/radioroom/down...o=file&id=1063
http://www.subsim.com/radioroom/down...o=file&id=1062
http://www.subsim.com/radioroom/down...do=file&id=730
Other than that all we can do is hope there is such a mod and someone who reads this knows about it.