Twilight is actually pretty crap at dark magic.
It feeds off the negative emotions and attributes of the user, so attempting anything beyond the ‘Spread Darkness’ spell would fizzle out pathetically.
Yeah, I can believe that. Dark magic feeds off of anger, fear, and, in a pinch, ambition (it may feed off of other strong emotions, but these are the Big Three)
Twilight doesn’t get angry often (not like resident hothead Rainbow Dash), Twilight knows deep down her fears are completely irrational but she can’t let them go (so while she does have fear, they’re about as useful for dark-magic-fuel as toothpaste in a gas tank), and Twilight is perfectly happy as Celestia’s student. Heck, her ascension to Princess came almost literally out of the blue (yes, she finished the spell Starswirl never could, but that was just her cleaning up her own mess, not out of any sort of pride or over-confidence)