In 1.8 pre-1 and 1.8 pre- 2 there where additions made to the colors of map.
I used TexelElf's "Map It" filter to create the maps in the picture below.
Now the map color for Black is wrong.

Most likely the developers are only thinking that maps are used for terrain, not custom maps.

TextlElf's explanation
https://twitter.com/Capt_World/status/524535944871813121