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Headcanon time, because of course, I need to ramble.
Dazai's hair was black during his time in the Port Mafia, but not because it was his natural color. It was a reflection of his own neglect. He didn’t take care of himself, rarely going outside during the day, and when he did, it was only in dark alleys. His hair was greasy, unwashed, and full of dirt, sticking to his scalp. God knows how much old dried blood remained in it as well...
It was only after leaving the Mafia that he began somewhat caring for himself, even if only a little bit. He still struggled with it (God knows depression was hard on him), but it was still better than his days in the PM. It was an improvement. Over time, his hair actually grew quite healthy, regaining its softness. The black color was gone, replaced by a warm shade of brown, and his curls regained their natural bounce, framing his face.
He actually looked healthy.