There are two types of Keyblades. Keyblades of heart and Keyblades of world's hearts. Keyblades of world's hearts have two subcategories: Keyblades of light and Keyblades of darkness. Keyblades of heart are able to unlock people's hearts and manipulate them in certain ways, and Keyblades of world's hearts are able to unlock/lock the hearts of worlds.
The only confirmed Keyblade of heart that we've seen so far is the unnamed one that was created by Ansem SoD using the 7 Princesses of Heart. All the rest of the Keyblades we've seen are Keyblades of world's hearts, therefore don't have the ability to unlock and lock hearts of people. But, Xehanort's Keyblade is able to do this. Doesn't this mean that his Keyblade is a Keyblade of heart?
EDIT: This question has been answered by Isamu_Kuno. If you'd like to know the answer, scroll down and read his comment. Thank you for the responses!