In the photo, which the Cruel Intentions star posted on Monday, she is sitting with her legs crossed and her hands drawing her feet towards her body.
Blair, who recently shaved her head with the help of her eight-year-old son Arthur Saint Bleick, is pictured next to a pool, wearing a red asymmetric bikini and stacks of bracelets and rings by Jemma Sands, which she calls a “summer staple.”
The 47-year-old captioned the photo: “Sunday. Thank you. And thank you @jemmasands for my bracelets and rings. Summer staple. And my always favorite @eres bikini [sic].
“Happy mum. Hugs to you all.”
Several of the actor’s famous friends and fans have since flooded the image with comments of support and adoration.
While some referred to the actor as a “warrior”, others praised her for sharing her battle with the neurological condition.
Grey’s Anatomy star Ellen Pompeo commented: “Looking like the glam warrior you are... superhero”, while singer Miley Cyrus added: “So pretty”.
Another person wrote: "You are fighting your battle in the public eye, a very challenging battle, all while living your best life with your handsome boy.
"You got this. Keep glowing and growing."
The NHS states that MS is a condition that can affect the brain and spinal cord, causing a wide range of potential symptoms, including problems with vision, arm or leg movement, sensation or balance.
It's a lifelong condition that can sometimes cause serious disability and is most commonly diagnosed in people in their 20s and 30s.
Earlier this week, Blair shared that she was having trouble sleeping due to the latest stage in her recovery.
“Insomnia. I am like a waking baby. Afraid and want to cry,” she wrote alongside a photograph of her bruised legs as she sat in a bathtub.
“I want my mum. I do. I will take a bath. And cry. The beginning is hard. I have to remember.”
Last month, the actor marked the end of a round of treatment for her MS with a celebratory photo of her newly shaved head.
In the photo, which the actress shared to Instagram, she smiles as she prepared to leave her hospital room with the help of an Alinker walking bike.
The star captioned the image: “Today is a banner day. I am being discharged from the care of an incredible team of nurses and techs and a visionary doctor who believes in my healing as much as I do.
“This has been a process. And will continue to be one. I am immunocompromised for next three months at least. So no kisses please. I wanted to make sure any complications that might arise here were my private space. And we got through brilliantly.”