Be expressionless, appear dumbstruck or overwhelmed. For example, When I asked her how to get to the hospital, she looked blank. [c. 1700]
Words nearby look blank
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.