Blue Adobe Slopes were introduced with update R36 on November 15th 2016.
2 of these Slopes can be made in a Processor from one block of Blue Adobe Wall each. No crafting recipe has to be unlocked for that.
The crafting recipe for Blue Adobe Wall is unlocked by finding the rare Recipe for it in a Treasure Chest or maybe obtain it from a Thing that spawns on corrupted blocks.
8 Blue Adobe Walls each can then be crafted with 2 (blocks of) Adobe Bricks, 4 blocks of any kind of Canyonstone (like Ruddy Canyonstone, Dark Canyonstone or the rather rare caramel-colored Canyonstone), plus 2 Stone Rods made of Stone, Limestone or Bedrock in a Processor, 1x Melted Wax made of Beeswax in a Forge or found in Treasure Chests and 1 Blue Pigment made from Blue Flowers (from Elderwood trees or Wood Treasure Chests) in a Processor.
Since update R43 in May 2017, inner and outer corners blocks can be created for Blue Adobe Slopes too by putting them into a Processor. No crafting recipes are required for this.