Ao ni, Fureru.

Ruriko Aoyama is a high school girl who has a blue birthmark on her face. She always tries not to care about it and be worried by others. One day, she happened to see her new class teacher Kanda's notebook. The pages were closely written about students' information in his class but Ruriko, and decides to question him about it. However, he reveals that he has a face blindness (prosopagnosia)—he can't tell people's faces.

(Source: Futabasha, translated)