Board Thread:General Discussion/@comment-26876292-20151012185355/@comment-8986886-20151012204043

Aizen did not create the skies over Hueco Mundo. He had the gigantic fortress called Las Noches built, which is located within the realm of Hueco Mundo. The inside of the large dome that surrounds Las Noches appears as a blue sky, rather than the eternal night of Hueco Mundo.

In chapter 263 Aaroniero claims that Aizen created the artificial blue sky.