Kagami's hair isnt blonde to begin with.. Probably pinkish.. just like the cover..
Yuujin (Eugeine ) has blonde hair where as Rio and Kagami have strawberry blond hair.
Masa was asking if Rio looked more like his father (yuujin) and Shingo says no that he looks like his uncle but the way he said it was in a indirect way of referring to kagami
http://www.mangago.me/read-manga/kuroneko_to_kareshi_to_ouji/uu/kuroneko_to_kareshi_to_ouji-chapter-1/24/