His Cutie Mark, while a bit complicated, stands for the dichotomy between the aspects of the dark, the lighter blue symbolizes the harmless and even friendly critters and "monsters" that come out at night, where the dark, almost black, blue symbolizes the Less friendly if dangerous and deadly things that come out at night. He likes to study and investigate these things, try to protect others from things that creep around in the night. In the center, the darkness is trying to push and take over the lighter, but the light tendrils are stopping the spread from the darker tendrils, a sort of balance of the two.