A character’s clothes can speak volumes about who they are. For instance, if someone appears in a book or television programme wearing dark clothing (particularly black leather), you just know they’re going to be trouble. Spike, Derek Hale and Negan are perfect examples of this.
Isabel Monray likes to wear black because she wants to give the impression of someone you do not want to mess with. Of course, anyone who actually knows her will see right through this.
She doesn’t want to appear vulnerable or afraid, even though she is in a very vulnerable position. Someone tells her if she’s not afraid, she’ll be all right; interestingly, that someone is wearing leather when she says this. But wearing the right type of clothes to show off your confidence doesn’t mean you actually are confident.
Later Isabel starts wearing her black jacket because she wants to wear it, not because she feels she has to.