When her father is assassinated, a sheltered princess searches the land to learn magical arts from her exiled mother to take revenge and save her people

The blind?daughter of a sorceress queen must search far and wide for her banished mother to learn from her?the magical arts- her only? chance for revenge?against the mighty?Evil One who brutally assassinated her father - The King.

This concept has been posted many times and I believe it's already received these same comments. The goal of revenge is a negative one - even the Count of Monte Carlo realized at the end that revenge alone is not worth pursuing...

What positive goal can you give her? As previously suggested, a good one would be for her to free her people from an evil/bad ruler.

Also as previously mentioned, including the antagonist, in this instance, is necessary and will provide an elegant way to specify the genre. If the antagonist is a sorcerer, she needs to learn magic and the story is supernatural, however,? if he's a brute and she needs to learn to be an assassin then it's an action thriller and so on.

"What makes the usurper so potent, so sinister she must resort to such extreme measures as sorcery?"

This is a great point that I hadn't thought?about. What exactly makes her the one who needs to be the protagonist? If this assassin is so bad, why wouldn't there be an uprising to take back their kingdom? The protagonist's little journey to go learn magic and then come back, would be useless if the people have already turned against the new ruler. Sure, if it's a powerful sorcerer(something we've seen all too often...just saying) then he/she won't have a problem keeping the power through fear.
Personally I think it would be much more interesting for this antagonist to be truly not evil at all. He/she genuinely wants to better the kingdom, free them from what they see as a bad reign. You could introduce the question as the princess is on her quest: Is she wrong? Is the assassin truly making the place better? As she travels she sees people completely happy,
I think that could be a much more interesting angle. I mean, sure, girl's father dies, she learns magic and something about herself and then she overthrows the bad guy and blah blah blah. We've seen it a dozen times before. At least with a male protagonist, but anyway, what is she truly fighting for? Because if it's for herself, for revenge, and she's wrong, that could end up a great character arc and an interesting antagonist who isn't just evil for being evil. That's not to say that the assassin is completely great, maybe the protagonist is actually right, and manages to prove it, or something.

That's just my opinion as someone who watches/reads a lot of fantasy and I've seen quite a lot of the same plot over and over again.
I hope this helps.