This can be learned by touching a Lorestone located behind Hrungnir of the Frost’s throne.

This will teach you Frost Smithing, but be ready: You’ll have to get through Hrungnir of the Frost first, and he is not happy to see you. Once you learn Frost Smithing, you can access the Forge of Ymir to start crafting with it. Black Ice is used in many crafting recipes. It makes the Black Ice weapons, requiring 20 of the substance for non-Ammunition type weapons. You can also make the Pride of Aesir set with Black Ice, Star Metal Bars, Alchemical Bases, and Perfected Medium Padding. Other than weapons and armor, you can use the ice to smith Hardened Steel Bars and Star Metal Bars. You can also build some rather stylish decorations for your base.