Ms Black, Ms Blue and Ms Green - Is there really a unique answer?

As requested in the comments:

Gardner's version of the puzzle ends with a line which is omitted here: $$\text {"So what?", said the lady in black.}$$

Note: a reference can be found here.

Somewhat surprisingly, that changes everything. It adds the information that Miss Blue is not wearing black (as she wouldn't have replied to herself, nor would she have disparaged her own observation). That makes the solution unique.

Note how delicate this information is. Had the line been spoken by Miss Black, we'd have learned nothing. Had it been spoken by "the lady in blue" we'd have learned nothing (as we already knew that Miss Blue was not wearing blue).