On Friday, the "Cruel Intentions" actress, 46, took to Instagram, where she revealed she'd been experiencing a particularly "rough day."
"Here’s a truth. I feel sick as all hell. I am vomiting and all the things which are not polite to speak of," Blair wrote next to a photo of herself snuggling up to a stuffed animal in bed.
"My son ran away. From me. I have to get him to school. The medical treatments take their toll," she continued, vowing, "I am going to get through this. We do. This will pass."
Blair also included a sweet message to other parents who, like her, are caring for their children while struggling with an illness.
"I hold you," she wrote, adding that it was hard in that moment to imagine "ever feeling ok again."
For the past six months, Blair has used social media to talk frankly about the challenges of MS, and to give thanks to friends and loved ones for their support.
One special friend she's expressed gratitude to is actor Michael J. Fox.
"I like this man," she wrote in her caption.
"Your presence," she gushed to Fox. "You. The best. Thank you."