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.
It is suggested that the Blue Hour only occurs in Bixby and its surrounding regions because 36 (as a multiple of 12) has strong connections to the Darklings, as does 96, and the sum of their digits equals 24, another multiple of 12.