in what chapter the blonde girl fell for the brunette? i remember the date chapter in the beach restaurant, what happened after with this couple?
That date was back in chapter 47. Aki was unsure if she was into Shiho romantically or not but they start dating anyways with Shiho proclaiming that she'll work hard at being a girlfriend Aki falls in love with.
Which seemed like we would be getting an arc of them being quasi-girlfriends like Yori and Himari started out as and Shiho needing to win her heart. But then when Aki gets home after that date, she finds her copy of the photos she and Shiho took a year+ ago. She recalls the day they took those photos and came to the realization that Shiho is actually really cute. (This happens in the volume 10 bonus booklet. Chapter number varies by site but is 49.5 here).
Aki never comes off as not being in love with Shiho after this in my opinion, but since they haven't talked about it on page, it's like she forgot about the whole quasi thing and thinks they have been full on girlfriends from the start. Which is really funny if true because that shows how hard Aki fell for Shiho.
Also in chapter 47, Shiho asked Aki to kiss her to prove she is serious. Only to then change her mind and told Aki to save the kissing until Aki really wanted to. And well, we all saw what Aki tried to do this chapter.