The actress naturally fed little Arthur after his birth last July (11) but was keen to gradually introduce powdered fluid into his diet as he grew.
However, she's been put off after reading a study which suggested that organic brown-rice syrup - a sweetener used in some baby milk - may be a hidden source of arsenic.
She tells TV Week, ""I was trying to supplement with formula, and that's the one they found arsenic in. It's very daunting, wanting to do the best for your child.""
Blair also admits she dreads filming Charlie Sheen's upcoming sitcom Anger Management because of the effect it may have on Arthur's feeding routine.
She adds, ""It's a real challenge. I have to figure out how to juggle it.""