Flawed question/answer?

一群人开舞会，每人头上都戴着一顶帽子。帽子只有黑白两种，黑的至少有一顶。每个人都能看到其他人帽子的颜色，却看不到自己的。主持人先让大家看看别人头上戴的是什么帽子，然后关灯，如果有人认为自己戴的是黑帽子，就打自己一个耳光。第一次关灯，没有声音。于是再开灯，大家再看一遍，关灯时仍然鸦雀无声。一直到第三次关灯，才有劈劈啪啪打耳光的声音响起。问有多少人戴着黑帽子？answer=3

I saw this question many times in different variations.
It seems that before the light is turned on second time, people who has black hat should slap their face.

For example, assume there are two people wearing the black hat. After the light is turn on and off once, there is no sound. This means everybody knows there must be more than one black hat. Then the two people wearing black hat should slap their face because they only saw one hat. Could you modify the question so that it makes sense?