I'm so confused. After finding u/Berzeck0 on reddit, looking through like hundreds of their posts I found the original. One comment said that it is from Yozakura Quartet: Hana no Uta. After watching the entire anime (which was really good imo), I found this clip in 10:33 of episode 10. However, Ao's hair is a bright blue, she isn't wearing a sailor uniform but rather a blue shirt, she isn't blushing as much, the window on the left is a solid rectangle (no divisions), there isn't a painting on the right and Hime's sleve is a similar shade of bright blue, not white.

This also explains why I wasn't able to find Ao on anidb; I searched for a character with black hair.

I thought that maybe there was another verion of the anime, but the only one I was able to find was Yozakura Quartet (without : Hana no Uta) with a completely different art style.

But I discovered a good anime, so I won't complain.