How bright will Betelgeuse become when it goes supernova?

A typical supernova of Type II ( massive star ) can brighten by about 18 to 20 magnitudes. Since Betelgeuse is a visual magnitude of +0.8, it will be perhaps a magnitude of -17 to -19 magnitude. This is brighter than the Full Moon ( about -18 ) but a bit fainter than the Sun ( -26m).

The bottom line is that for most of the year, there will be no pitch black sky for the better part of a year, because Betelgeuse will never be more than 12 hours away from the sun in rising.