wh question

[daisy1352]

He looks good in blue.
What color does he look good in? Is this a correct question?

My students are sharp. What is the opposite of sharp here? Is dull correct? What about blunt?

[MikeNewYork]

He looks good in blue.
What color does he look good in? Is this a correct question?

My students are sharp. What is the opposite of sharp here? Is dull correct? What about blunt?

Your question is fine.

"Dull" works, but some may think you mean boring.
"Blunt" has that definition, but some may think you are making a joke.
You might try obtuse, slow, weak, dense.