Instead of responding to her comment "I heard all about your little party", the "I" person here did not respond and probably showed no expression on their face, out of confusion. The "I" person does not seem to know what she is talking about, so their facial expression was "blank", just like their feeling. The feeling that makes you say "Huh?". So the sentence means that their face and their feeling were the same.

This is the same use of "blank" as in "I drew a blank" = "I could not think of anything", eg "When John was asked what his favorite movie is, he drew a blank, because he never goes to the movies" in the example, the person does not know what party Marsha means. PS - No one (in the USA at least) says "I feel blank" - the sentence is very unusual

Three things are happening here. 1) The person narrating felt "blank." It could have different meanings. a. The person meant they feel empty inside. Emotionally empty. For some reason, they cannot feel emotion. It is also possible that they do not care, so they feel nothing. b. They are "spaced out." It means. Their mind is not focused on what is in front of them. Maybe they are thinking of something else. Maybe they are so shocked they cannot think. Maybe the narrator does not understand what the other person is talking about. c. We can only guess at the reason. It depends on what the rest of the story says. 2) This "blank" feeling is showing in the narrator's look. How the narrator appears: their facial expression, their gestures. 3) "I must have looked" implies that the narrator is imagining what they look like to the other person. The narrator does not have a mirror to check their appearance. So the narrator is guessing what they look like.