An old picture frame, painted and sanded. Blackboard paint. Old wallpaper.
Paint a blackboard on the wall, just slightly larger than the inside of the frame.
Tear old wallpaper into decent sized chunks and paste around the board. I used two different patterns as I wanted to give the impression of layered, peeling paper.
Stick the frame onto the wall.
Ta Da! One kitchen blackboard window.
(N.B. Be sure you really like the place you've chosen to create your window before using 'Hard as Nails' to fix your frame. I've just been reliably informed that if I ever decide to move my window I'd better start practising my plastering technique; it's likely to take a large chunk of the wall with it....Whoops!)