Many fans praised the bold image, including a handful of celebrities.
Modern Family star Sarah Hyland responded to the Hellboy actor with smiling emojis, while model Jaime King wrote, "I love you cutie booty."
And Blair's Cruel Intentions co-star Sarah Michelle Gellar joked, "You know they have medicine for that," in reference to the crab purse.
Others, however, expressed disapproval.
"Wear your pants," one commenter demanded, to which Blair responded, " I don't have any. Can you believe it! Minimalism." Another simply wrote, "Sorry unfollowing," to which Blair replied, "Is it something I said?" When another "didn't get it" and questioned the motive of the photo beyond "shock factor," Blair responded: "I think it's a vulnerable, humorous, odd and interesting fashion shot. I like it."
Blair also responded to the negativity with a follow-up post depicting a Barbie doll in the same pose.
Blair was diagnosed with MS almost a year ago and appeared in public this year walking with a cane.
Last month, Blair revealed her treatment left her immune system compromised, and she experienced hair loss.
She has posted several photos to her Instagram account since to reveal the extent of her hair loss.
In a cover story for People magazine earlier this year, Blair opened up about her MS battle, saying she has experienced "constant weakness and fatigue" and spasmodic symptoms that compromise her control over her face and voice.
Blair, who has an 8-year-old son, Arthur, first shared with the public how MS had affected her in a revealing interview with Good Morning Americain February. During that interview she was dealing with an "exacerbation" of MS — resulting in spasmodic dysphonia, a condition that alters her voice.
