Military Origins: Spec Ops
Perhaps this is not a commercial application, but most innovations start off with military applications, and then work their way into normal day to day life.
The darkness generator is the newest weapon developed by the army. Against a technologically underdeveloped or unprepared enemy, it provides a massive advantage by completely blocking off all vision as well as communications equipment (by also blocking off electromagnetic waves).
A specially trained Darkness Squad can set up the generator around their target, and have their own portable sonar devices and dead reckoning systems to guide them in the darkness. As @Matthew pointed out, they can communicate among each other using modulators and demodulators for their voices using sound frequencies beyond the range of the human ear (ultrasonic or infrasonic transmitters and receivers) which can be encrypted and decrypted for added security. Specially created weapons that fire based on sound cues, and melee weapons would most likely be their go-to method of attack.
With the Darkness Squads' superior communications and sensors, the enemies plunged into darkness and blocked off from any form of communication don't stand a chance.
An area the size of a football field is more than enough to shut down operations and communications in an entire building, and it makes it extremely difficult to send in reinforcements, due to the communications block off and lack of visual confirmation in the generator's area of effect.
Commercial Use: Agriculture
Eventually, the world will have seen enough use of the generator, that easy ways to counter it will be found, discontinuing its usage for military purposes. Instead, it is now used in areas around the world for the agriculture industry.
Farming is one of the applications of the darkness generator. By employing the darkness generator, it is possible to fine-tune the amount of light any farm receives. This would be especially useful for areas that are too hot to plant certain colder-climate plants.
Farming in the desert could be achieved as well, as long as the soil and water conditions can be provided, the amount of heat and sunlight can be fine-tuned through use of the generator.
Commercial Use: Space Colonies
Perhaps the most useful application would be the use of the darkness generator in space colonies on the outer planets.
By using the generator on habitation areas of a planet, it is possible to bring down the amount of sunlight, and thus heat, in certain habitation areas, bringing down the total temperature. Of course, this will not reduce the heat brought through the air from surrounding areas, but should help alleviate some of the heat wave from the ground in the generator's area, which will make otherwise uninhabitable areas much more bearable for habitation.
Also, with the blocking of electromagnetic waves, the darkness generator could drastically reduce the amount of radiation humans experience while on certain planets, such as Europa, where the radiation is enough to be fatal in just one day (although blocking off just the sunlight for temperature related purposes would not be useful, considering Europa's temperature is something like -170 degrees).