What methods can be used to make a dark colored window become opaque?
During the daytime, the daylight outside reflects in the window and drowns out the light that comes from inside. The darkening of the glass only reduces the light that comes out the window, and at night there is no reflection to drown out the light, so the light from the inside can still be seen although it's reduced.

At night you have lights on inside, which of course increases the light that comes out. Dimming the lights will reduce the amount of light that gets out.

So, there isn't much you can do, unless you install massive floodlights to light up the surroundings at night, to get the same effect as you have at daytime.