What you purchased was an Athame and should be named as such. There are tabs in your backpack, one of them is for Athames. It does not show up in your hand as an Athame is not for battle use. The Athame's primary use is to direct energy which is why your Athame will increase your stats in some way.
Swords on the other hand, are listed under wands and act like a wand, which means they do show up in your hand and grant spell cards.