The black areas of the map are areas that neither you, nor your allies have explored. This blackness is called the black map. As soon as you send units to explore these black areas, the black map will be replaced with the <link="topic:fog of war">fog of war</link>.

Black map is related to <link="topic:fog of war">fog of war</link> and <link="topic:line of sight">line of sight</link>.