There was a Skill Sphere at Home and another in the Via Purifico, so I guess you got one of those two. You can bribe Zu's for 2 Skill Spheres, but you won't be able to get back to Bikanel for a while. So you'll have to do without until you get the airship. But does Yuna know any Black Magic? Because if she doesn't then that skill is pretty waisted on her until she knows some.