Analog Vref for ADC on STM's Blue-Pill
There isn't one in a 48 pin package, it's too small to have extra functions like that. Even the 64 pin doesn't have one, only when you get to 100 do they include the ref pins. 


They do have an internal reference, you'll have to configure it. 


Sources: https://www.st.com/resource/en/datasheet/cd00161566.pdf