Ladies: I don't know what a black signature means during the day and night.
What she means is that she is not from the same world as some people, and some people don't understand her, just as they don't understand the darkness at night during the day. She used this sentence to express that no one understood her, and no one understood her loneliness and helplessness. This sentence may also reflect her loneliness. This is my analysis, which may be incorrect. If it feels good, you can adopt it.