I think the viewing you are referring to is the one from Chapter 51 of Towers of Midnight:
Rand reached out, touching the weapon. She glanced at him, and a viewing sprang to life above him. A glowing sword, Callandor, being gripped in a black hand. She gasped.
“What did you see?” Rand asked softly.
“Callandor, held in a fist. The hand looks to be made of onyx.”
“Any idea what it means?”
She shook her head.
“Rand,” Min said, taking his arm. “My research…remember, Callandor seems to have a deeper flaw than we’ve discovered. This viewing only reinforces what I said before. I worry it may be used against you.”
We know that Min's visions are often fairly vague, and she only claims that they come true for certain when she knows what it means. She also doesn't say that the hand gripping the sa'angreal is Rand's.
I take this to be a reference to the Last Battle, where Moridin (in truth the Chosen Ishamael) -- not Rand -- takes hold of Callandor. The "black hand" being symbolic of Moridin's position as Nae'blis.
From A Memory of Light, chapter 46:
Moridin dropped his sword, and stabbed his own right hand with the knife. Rand twitched suddenly, and Callandor dropped from his grip as if his hand somehow hurt from Moridin’s attack.
The glow emanating from the blade winked out, and the crystalline blade rang as it hit the ground.
Moridin scooped Callandor up off the floor. It burst alight with the One Power.
Rand stumbled away, holding his aching hand to his chest. Moridin laughed, raising the weapon high. “You are mine, Lews Therin. You are finally mine! I…” He trailed off, then looked up at the sword, perhaps in awe. “It can amplify the True Power. A True Power sa’angreal? How? Why?” He laughed louder.
Here we learn that Callandor is a sa'angreal not just for the One Power, but for the True Power also -- showing the darkness and flaw of this weapon, that Min had sensed.