Celestia using dark magic... Man, this season is going to be amazing! (Although I can admit, this could have creeped me out when I was little. Can't really say how it will affect the actual kid audience)
"Now, Twilight, I'm going to give you a test, and if you pass, you get to move on to the next level of your studies, mwahahaha...Here, you'll need to know this dark magic spell to pass the test. All you have to do is channel your anger and hate, like this! ...Yes, good, my young apprentice! Let the darkness flow through your heart!"
The fact that Celestia could use 'black' or 'dark' magic was pretty neat, but it wasn't surprising for me. Why? Well, when you have to live for over a thousand years and you need to earn the right to be an alicorn with great magical powers to rule over Equestria with justice and peace, I'd imagine you'd have to know both 'light and 'dark' in order to condition one's self. The best ruler is a ruler who knows how 'darkness' works as well their own 'light.'