Rinoas powers weren't blocked or anything like that. That's not how sorceress powers work. Once you receive the powers, you have the powers, simple as that.

Now, what actually happens at the end of disc 2, when you defeat Edea, is that Ultimecia forces Edea to give Rinoa her powers. Ultimecia realised that with Edea quite beaten, she needed to use someone else instead. That 'somone' was Rinoa.

After receiving her powers, she fell into some sort of coma. This is either because her body needed time to get used to the new powers, or because Ultimecia had already started possessing her then, but needed time to get Rinoa fully under her control. Either way, Rinoa wakes up as a sorceress.