Perhaps. it's called Alicorm Magic not because it can only be done by alicorns, but because only Alicorns can use it safely?
Sombra used it and it drove him mad(My theory)
Celestia used it no problem
Twilight used it pre-Alicornification, but either one use isn't enough to corrupt, or potential Alicorns are close enough to full ones for no corruption, if My theory is right
And again, dark is not evil. All we've seen it used for is illusions and mind effecting things, and color changing one potion(Which may have been an effect of the potion, to show it's ready)
So, the superpowerful mind magic might make you go crazy if you aren't strong enough to control it, and aparently only alicorns are normaly strong enough to control it.
Add the fact that it was never called Dark magic in Canon, and was then called Alicorn magic right in the episode?
I do belive that it's supposed to be alicorn magic, officially