Bonjour Daniel ! That's a great question, and the answer is simpler than it seems :) Here it's not the woman who's black, but her clothes = in black (clothing). Therefore, the adjective won't agree here: "en + [masculine colour]". "La Dame en noir / en blanc / en vert..." It would be the same in sentences such as: "J'aimerais cette jupe en violet." (I'd like this skirt in purple.) I hope that's helpful! À bientôt !