Reference voltage for a comparator
Your calculation and reasoning re negative input terminal is correct. 

The comparator compares Vin with the reference voltage so Vin can be any legitimate voltage allowed by the datasheet. This varies with device but is typically about +/- 13V for a +/- 15V supply. 

The purpose of the comparator is to set Vout  to a value that indicates whether Vin is above or below Vout. You can determine what polarity Vout will have for various value of Vin.