It is implied that the Blue Hour only occurs in Bixby and the surrounding areas because 36 (as a multiple of 12) is a number strongly associated with the Darklings, as is 96, and the digits of these numbers added together is 24, another multiple of 12.
The presence of the Blue Hour in Bixby and surrounding areas is inferred due to the unique numerical connections between 36 and 96, which are associated with the Darklings. This connection also happens to result in a sum that is itself a multiple of 12.