Scenario - Map Options
Untick "Masked Areas are Partially Visible"
This makes the terrain completely invisible, however, once a unit explores the terrain, it will become partially visible. I think there are triggers which can fix that, somebody else will have to help you there because I'm not sure.
Sight blockers block the vision of units that are exploring the map. A line of sight blockers will prevent the unit from seeing beyond that line, kind of like how trees or cliffs block the sight of units.
Hope that helps.