The Blue Wood Wall is a blue wooden building-block. It can be crafted in your Crafting Menu (default key "Q") or at Crafting Tables (that still exist in worlds created before R22 on September 16th 2015), but only after the crafting-recipe has been unlocked upon taking/crafting a block of Wood Wall for the first time (which is unlocked when taking Cragwood, usually from a Cragwood tree). To craft 8 (blocks of) Blue Wood Walls, you'll need (as of R27 in January 2016):