There is also some symmetry in the way the 'dark' pipes connect to the blocks.

On the left and right they connect to the same 6 'mirrored' blocks, tough one pipe connection has a different orientation.
On the top and bottom 3 out of 4 blocks are the same, one is different.

Also taking the 24 blocks that those pipes connect to, there is an abundance of 'bottom-right' triangles: 13 of the 24.

[Edit: Color blindness yay... checked the sheet and looked again... the pipe at the top only connects to 3 blocks)