Black dots represent enemy controlled territory. You must take each black dot with an army. Fleets are unable to take black dots according to the rules amended at the Toronto Conference of 1993. Fleets may be used to convoy armies to black dots, or they may be used to ship black dots to armies; either way, they can take and move black dots but not control them. Similarly, armies are able to control black dots but cannot move them from territory to territory. To "control" a black dot, you must set up a chain of armies between your home black dots and a foreign black dot. For example, if you are France and you'd like to take Tunis, you must set up a chain of armies from the closest French black dot, Marseilles, to the Tunis black dot. However, there is water in the way! Armies cannot go on water. Fleets also cannot go on water unless carrying an army, because otherwise, it's just a boat with nobody driving. You'd need a total of six units: an army in Marseilles, an army in Tunis, fleets in the Gulf of Lyon and Western Med, and armies on both of those fleets. You then order your army Tunis to "convoy" to Marseilles, thereby taking the black dot from Tunis and putting it on Marseilles.
The problem, though, arises when all the neutral black dots are taken. What then? You must "search" for new black dots, which can only be done with fleets. Armies can take but not search, fleets can search but not take. Try looking in territories like Ireland, Iceland, and Cyprus to find new black dots. Sometimes, black dots hide in surprise places, like the South Coast of Moscow or on different boards, like the Colonial or Rat Wars board. You'll need to go to VDip for those, so set up a different account and start a game over there. I hear it's much better anyway. The goal of the game is to control 18 black dots before the other players find the Snitch.