Black’s dad owned a small studio/workshop across the street from their main house, on a hill in a forest-like area. (have to draw that area now lol)
Yaldar, after their dad left the house, moved to that studio. 

With time, Black started spending most of his time there rather than at the main house, and he eventually took a room in there. So he had two rooms growing up.

This is the one at the studio.

and yes, to avoid running into Yaldar, and because of the ridiculously various different times of day and night he visited Black, those big windows were his entrance.