I'll give it a shot:
To black-click, right click on an unexplored square. Then look at the info panel to your right, under the map. You should see:
Loc: (##, ##,) ##
The two numbers in parentheses indicate the grid coordinates of that square. However, this is not the "magic" of black-clicking.
Take a look at that third number. If it is:
1--It is usually an ocean square.
2-62--It is a land square on an island or continent. Each land square that is connected to another land square is considered part of that landmass, and all squares in the same landmass share this number. Different numbers indicate that the squares being compared are parts of different landmasses.
63--It is usually a 1-square "lake" surrounded by land squares.
Black-clicking is a very useful way to "get a feel" for the lay of the land before you actually explore it. Some consider it cheating, while for others it is a major part of their early game.
Hope this helps.