I'm sorry - but other then the obvious "frame" reference, I'm not sure why you chose this shot. There's nothing to focus on inside the frame. And the frame itself, is not very interesting. No true whites or blacks on the piece with which to make it even a nice lighting excersise.
It's a pity the frame doesn't "frame" anything. It's a neat idea for the challenge, but would have been nicer if there really was something in the frame.
I like the concept here, except there's nothing in the frame. If the clouds were in it, or maybe if the pic was in color... it seems a touch boring like this.
It is often difficult to get the perfect shot. If you were allowed more post-processing you could put a single cloud within the frame and leave the rest clear blue. This would then be perfect. As it is it seems like a missed opportunity. I really like the idea though.
This picture I simply love. For being so simple.
Did you try taking the photo from different angles? I would try closer towards left with around 45deg anlge so that the frame spreads from left-bottom to right-top. Just a suggestion. Maybe making it brownish instead of just B&W would give better feeling. Still 7