He gave her a blank look, as if he had no idea who she was.

他茫然地看了看她，好像不认识她似的。

《柯林斯英汉双解大词典》

There was a blank look on his face.

他脸上没有表情。

youdao

I tried to explain, but he gave me a blank look.

我试图解释，但他却茫然地看着我。

youdao

It takes a blank look and actually makes it look like a frown, and it takes a smile and makes it appear neutral.

FORBES: Smile For The Camera -Media Training

Whether that's what the 24-year-old star was aiming for is hard to say, but he sleepwalks through this tearjerker with a blank look and delivers his (admittedly awful) lines with numb detachment.

CNN: Review: 'The Lucky One' is an unconvincing romance

His wife, Zoe, then told the inquest she saw Bird carrying the rifle with a "blank look" on his face and was "unnerved".

BBC: Derrick Bird asked 'nice day?' before shooting tourist