Is it possible to un-learn crafting recipes?
No, there's no way to un-learn a recipe. But luck is with you! Every legendary has a level 70 equivalent. It'll be slightly renamed (ie. Umbral Oath has a level of 9, and it's level 70 variant is the Living Umbral Oath), but the sprite will be the same, and the stats it gives will be along the same lines, just modified for level 70.
So the level your character is at to teach the blacksmith is irrelevant. The plan will teach the same thing, regardless, but you don't need to worry, as every plan has a level 70 equivalent.