Meaning:

“黑” means “black,” and “化” means to “become.” This term, frequently used by animation and comic fans and gamers, refers to the drastic turn seen in a fictional character from a hero into a villain. The term originated from Japanese game “Fate/Stay Night.”

Example:

A: 听说很多《权游》粉希望HBO重拍第八季。

Tīngshuō hěnduō 《quányóu 》fěn xīwàng HBO chóng pāi dìbājì。

I’ve heard that many “Game of Thrones” fans hope HBO will remake the show’s Season 8.

B: 大概他们受不了剧情急转直下，小恶魔忽然不再睿智，龙母黑化的厉害。

Dàgài tāmen shòubuliǎo jùqíng jízhuǎn zhíxià，xiǎoèmó hūrán búzài ruìzhì，lóngmǔ hēihuà de lìhài。

I guess they cannot stand the plotline going in a downward spiral. The imp suddenly lost his wits and Daenerys became really bad and mad.