The actor, best known for her performances in Cruel Intentions, Legally Blonde and Hellboy, wore an ethereal, multi-coloured strapless dress by luxury fashion house Ralph and Russo.
She accessorised with a matching sheer cape attached to a choker necklace, and held a black cane as she posed for photographers.
The condition can affect the brain and spinal cord, leading to problems with vision, muscle spasms and fatigue.
"I am disabled. I fall sometimes. I drop things. My memory is foggy. And my left side is asking for directions from a broken gps. But we are doing it," Blair wrote on Instagram in October.
The actor appeared visibly emotional on Sunday evening as she made her way across the red carpet at the annual Vanity Fair bash.
"It took a lot to come out here," she said, as the room of photographers cheered her on.
Blair's cane was customised by celebrity manicurist Tom Bachik, who adorned the handle with the actor's monogram and a pink diamond.
"How did I get so lucky? I wanted a special cane for [the] #VanityFair dinner," Blair wrote on Instagram.
"Hours of love put in," she added. "And then #tombachick made it especially magical. I burst into tears. These gifts to get me through."
During the festivities at the Vanity Fair afterparty, Blair had her portrait taken by photographer Mark Seliger, who made her billowing cape the focus.
"When my life flashes before my eyes, I want this portrait by @markseliger to be front and centre," the 46-year-old wrote in an Instagram caption accompanying the photo.
People have expressed their admiration for Blair's ensemble and for raising awareness of MS.
"What a powerful image! You did so much for so many, this evening... and you looked stunning!" one user wrote in the Instagram comments underneath the portrait.
"Breathtaking. Love it. You look flawless #warrior," another added.
For a full list of all the winners at the 2019 Academy Awards, click here.
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