If your head is empty, you are fundamentally stupid and ignorant. There is no information inside your brain. This is usually a fairly strong insult. "Never ask Bob for advice. He is an empty-headed moron." We don't say your "head" is blank; we say your "mind" is blank. If your mind is blank, you suddenly cannot remember a single, specific fact, or you cannot perform a single, specific process RIGHT NOW. A: "What's Bob's wife's name?" B: "Uh...Uh....Wow! My mind just went blank! I've known her for several months, but I can't remember her name right now!" When the teacher made little Sam do the multiplication problem in front of the entire class, his mind went blank. Sam was usually very good at multiplication, but, when all the students in the class started looking at him, he suddenly forgot how to multiply.

Hi ChildoYaHUah. You explain that well. In fact, now I think a blank mind is an usual phrase rather than a blank head. Two examples I have found: "Well, what about me? Sat at home with a blank canvas and a blank mind." "An empty purse is much better than an empty head."

I haven’t heard of those saying that you mentioned in the subject bar before, if I’m not mistaken. But I looked up what you mentioned in the subject bar, a blank head is referred to as brain fog. I guess in relation to not being able to think clearly. And a empty head means lacking seriousness, intelligence or knowledge. Examples: Susan draws ‘a blank head‘ when it comes to executive responsibilities. Aaron surely has ‘an empty head’ whenever a woman gives him a compliment about his looks. Does this help?