I was talking figuratively about the lightness and darkness of magic usage. Twilight has already helped restore the balance between night and day by wielding the Elements with her friends against NMM.
 
But on the note of Luna’s abilities:  
Luna’s magic is unlike what is usually seen by other magic users. She can shapeshift, animate and radically alter the appearance of inanimate objects, and conjure a literal storm in the sky. Luna’s magic abilities have the potential to be greatly misused and cause great harm - even oppress an entire kingdom.
 
However, ever since her NMM corruption was purged, she has been a ‘good’ pony (I personally believe she’s closer to neutral, but angles toward ‘good’ when a great evil emerges). Perhaps her NMM corruption was caused by her trying to find ways to wield her powers in a harmful way.
 
Meanwhile, Celestia is ‘very good’. While it’s obvious that she has done horrible things in the past and has skeletons in her closet, she does what she can to be an example to others. At the same time, she’s also demonstrated that she can wield the same ominious magic that Sombra uses, and yet remains unaffected. Perhaps the frequency of usage is key, which is where striking the balance is critical.
 
Twilight could possibly strike a balance between the light and dark arts of the arcane. She’d run less of a risk to herself and others by finding a balance, and being versed in both accepted and taboo forms of magic would make her a fierce opponent, since a wholly evil threat’s spells would be recognised by Twilight and more effectively countered. Plus, Twilight would have less reservations with using darker magic to protect others.

No magic is inherently good or evil, but their effects can act as a catalyst for corruption, especially dark magic.

@koko Well, she is the goddess of the night.

So what Mattatatta, then Luna is 100% darkness?

Well, Twilight is what occurs between Day and Night, this means there is a marginal presence of darkness and light at the same time.
 
For Twilight Sparkle, this could mean that she must learn to harness both “light” and “dark” magic in order to establish a balance, a harmony, between the two opposites.

“Dark” magic sure does cause a headache.  
Ow!

I somehow get the feeling that this is only the beginning. That is… Twilight will fall to absolute darkness and will need to be saved by her friends before she gives herself completely to the shadows.

I was actually a little shocked to see her trying what passes for “dark” magic.