Obtuseness, n.
State or quality of being obtuse.
Shades of meaning
Fernald has no entry of its own for obtuseness; he lists it under stupidity.
Where he writes about it:
Stupidity is sometimes loosely used for temporary dulness or partial stupor, but chiefly for innate and chronic dulness and sluggishness of mental action, obtuseness of apprehension, etc. — from stupidity