Dark.
Blonde is nice until the novelty wears off. Blonde is too distracting, dark hair let's you focus on all her greatest features, her round baby eyes, her squishy cheeks, her bunny teeth, etc.
I agree! This is actually what I was thinking. There's a big difference between Nayeon with dark hair and with light hair. The dark hair makes her features pop more and brings more attention to them. The blonde hair makes her features more washed out but not in a bad way. Just not as prominent.