My interpretation is that she had an unrequited love for black hair, but was trying to keep the three of them together. Blondie kept coming on to her, but she rejected him and firmly kept the three of them platonic because that was the only way she knew how.
It seems like she had a slight thing with black hair while blondie was gone (the thing about the passenger seat being her spot). She's changed over time and is more accepting that things can't be the same between them (i.e. not wearing a yukata to a festival because of her brother teasing her about meeting a guy vs. wearing a yukata to the festival as a grown up). Things get heated with her and blondie, but in the end it turns out he has a gf. She feels like one of their trio is being stolen away.
Blondie asks her why she's not with black hair and she says that it's because of "this feeling" and they both look a little sad. Maybe because they feel like they're ruining their friendship by splitting off as a pair. But in the end, she says she "likes it here" as in their trio, and blondie agrees. She breaks away from blondie to rejoin black hair and make their group of three whole again. Black hair seems to notice something was up from the way he looked at her but doesn't say anything.