Now that we have some chapters with Tomikawa having black eyes, I would like to give my theory as to what it means.
First off, given that most characters in the manga have black eyes (an exception is the hairstylist), I think it mostly just shows off the general colour/shade of their eyes rather than the people being a certain way. So a possibility is that she just removed her contact lenses, but since her eyes have been shown to shift back and forth and there isn't much of an implied space between chapters 71 and 72, I think it reflects something specifically with Tomikawa.
So what does she do differently when she has black eyes? Well, she thinks about her family situation, her past, her relationships. (She is also shown to have black eyes in her past in chapter 85.) With her "shaded" eyes, she doesn't do that. She appears to mostly think about things like having simple fun together with Hiiragi. So I think her "shaded" eyes show when she is mentally shielded and that she has been shielding herself from thinking about how her life sucks by detaching herself from the more serious topics of her life. With the seemingly permanent shift of her eyes in chapter 70, I think it means that Tomikawa from that point began approaching things with those heavier topics in the back of her head.
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