I didn't get what "mistook for simple-mindedness the surface of artlessness" means.
Usually, we'd say that you mistook the brown candy for the blue one. Meaning that you actually wanted (or expected) the blue candy but instead (may be mistakenly) picked the brown (wrong) one.
Applying the same analogy here, does it mean that the critics (wrongly) interpreted artlessness as simple-mindedness?
Here is the complete sentence as it appeared in a reference material for an exam prep:
"Early critics of Emily Dickinson's poetry mistook for simple-mindedness the surface of artlessness that in fact she constructed with such ___."
Now depending on the (correct) interpretation of the sentence, I would construe a positive or negative word for the blank. Example, the poet's work was good or naive.