Actually, the word "obtuse" is most certainly and correctly used to describe a person who is ... well, obtuse -- blithely and persistently indifferent to, or unable to grasp, concepts or information. For example, if you try to explain long division to a student who just doesn't get it no matter how many times and how well it is explained to him, you can quite correctly describe that student as "obtuse."
One of its dictionary definitions is "slow to understand."
I've used the term myself on FR when dealing with obtuse people. However, Windflier is most emphatically anything BUT obtuse. Windflier is as sharp as a tack, IMO. He's been one of my favorite FReepers for quite awhile.