Oh! I don't know if I have faceblindness, but I definitely can't do that. Trying to picture anything in my mind, invented or remembered, is like a very near-sighted person trying to squint at a thing across the street -- the details just aren't there to see.
When I did a journal comic, which required drawing people who looked like themselves, I had to either have photos of the person in question, or take serious note of specifics while I was looking at them: "okay, he's got a pointy nose. He has big eyebrows" etc, because I won't be able to recall any of the specifics when I'm not looking at them. All I remember is the general feel of the person, so I can usually tell when I've got a decent likeness.
But it never struck me as something special with faces. I don't remember their hair or clothes or whether they had glasses, either, even if it's someone I see often or just finished looking at. This used to drive my mom up the wall -- "You had a new teacher in school? What did she look like? What colour was her hair?" "I have no idea." "How long was it?" "I have no idea."
........okay out of curiosity I looked up "signs of prosopagnosia" though, and....... I can tick off most of the list. Not recognising people out of context, failing to recognise people when they change their hair, inability to keep up with multiple characters in a live action show or movie, being nervous about the inability to keep classmates straight, all apply to me. SO UH!!! MAYBE??? Or maybe my brain just doesn't store visual information period??? MAYBE BOTH!!
Cartoons are much easier for me to navigate visually than real people, though!
I know my characters, and I don't have trouble assembling their faces and making sure they're on-model unless I haven't drawn them in a long time.