You need a wizard hero in the kingdom...and who has levelled up a couple of times so they have the ability to craft it unlocked. Once this has happened, your Blacksmith can find the Wizard in game, and buy a potion off them by using the 'Purchase Crafts...' interaction.
Another way of doing it is by actually crafting the potion when playing the wizard, finding the blacksmith sim in the world, and giving them the potion. You do this by dragging the item you want to give out of the playable sims inventory, and letting go when the cursor is over the sim you want to give it to.
And just so you know, later on the Blacksmith will learn recipes that can only be made with materials gained through trade with the other countries...so you will need a merchant. It gets very complicated.
Where's the Kaboom? There was supposed to be an Earth-shattering Kaboom!