From Leopard Gecko Wiki
The Midnight Blizzards are a phase of the Blizzard morph. They have yet to be proven genetically. What many breeders have classified as Midnight Blizzards, are Blizzards who have darker coloring than other Blizzards. Some feel the coloring comes from incubation temperatures, others believe the colors come from cage temperature. With Leopard Geckos, the lower the temperatures, the darker their coloring seems to be.