The Blue Concrete is a hazy blue building-block.
It can be crafted in your Crafting Menu (default key "Q") or at any Crafting Table in worlds created before R22 on September 16th 2015, after the recipe has been unlocked upon taking a (block of) Siltstone for the first time (like by mining it from the Stalactite layer with at least an Obsidian Mining Cell or harvesting from a Warmworm-pet).
To craft 8 (blocks of) Blue Concrete, you'll need (as of R22 in September 2015):
- 1 Blue Pigment made of Blue Flowers from Elderwood-trees in a Processor or found in Obsidian Treasure Chests
- 1x Melted Wax made of Beeswax in a Forge or looted from Wood Treasure Chests or Stone Treasure Chests
- 4 (blocks of) Siltstone from the Stalactite layer
- 2 Globs of Goo (made from Mold in a Processor or looted/harvested from many creatures including Pigsies