Pick blocking blocks will not work if the block you want is inside your backpack but not inside your inventory.

STEPS TO REPRODUCE

1. Equip a backpack and put a block of any kind in it. Make sure that block is not inside your normal inventory
2. Place a copy of that block and middle-click to pick block it.
3. Notice that nothing will be placed into your inventory.

Not sure if this is fixable, I'd have to look into it

Some other features in a similar vein to this: Jei + recipe book moving items should also select items from your backpack.

Also, the recipe book will not consider stuff in your backpack