I agree with Troy. "Blank face" or "blank expression" is how we would describe this in English. The phrase "poker face" implies that a person is purposely concealing their emotions, while a "blank expression" or "blank face" may be on purpose or may not be.

In English, it is called a "blank face." If they do not seem to be able to look in more than one direction for a prolonged amount of time, that is called a "blank stare." A "poker face" is a face without emotion. It is derived from the use of a poker face when playing poker, a card game of skill and luck. When one has a poker face, his opponents cannot determine if he/she is playing a good hand (the cards are favorable) or a bad one . A good player will use a "poker face" that will not show the regret of a bad hand or the excitement of a good one. A "poker face" is deliberate (made on purpose). A "blank face" is not necessarily done on purpose, but could be a result of traumatic emotion being withheld deep inside.

也许你的意思是沉闷