1. What does it mean?
흑화 literally suggests turning dark. In character discussion it describes a change toward a threatening or bitter side; in everyday joking it can exaggerate becoming cynical after frustration.
Also written or said as heukwa
2. The nuance
It is not a mental-health diagnosis or a precise narrative category. A temporary angry mood and a lasting character change are different, even if both get called 흑화.
3. Examples
“늘 밝던 주인공이 배신 뒤에 흑화하는 전개였어.”
The normally cheerful protagonist took a darker turn after the betrayal.
“게임에서 계속 져서 흑화하기 전에 쉬기로 했어.”
I kept losing, so I decided to take a break before getting bitter.
Sources
References support the meaning and usage. Example sentences are written for NoCapWiki.