Once started, I'll change the lights, if necessary, to help create the scene. It's difficult to drop an idea and go with something new invisibly as the tech person, so make sure your choices support a scene and don't dictate it. Lighting can establish scenes in countless ways, a cavern can be implied by dimming the lights, an underwater scene can be supported by blue lights, a German expressionist film can be implied by stark lighting in only certain areas. A sudden change in lights will be read as though it is part of the action and is supposed to be acknowledged by the actors and audience. A gradual change will seem more atmospheric.
Most scenes only require a general stage wash.
Don't try to force special lighting effects into scenes in which it isn't supportive. When the time for the blackout does arrive, it's best to end with as fast a blackout as possible for a comedic or thriller sorts of scenes. Don't start to take the lights out and then back off because the scene keeps going. Make the choice with confidence. Barring the need for a change of set or costume, or if filler music is being used, keep the lights blacked out for a moment to establish the end of what was just seen, but bring lights up again before the audience realizes that they're just sitting in the dark.