For RaspberryPi I found this: https://github.com/stephane/libmodbus but I'm trying to understand how I can implement a modbus RTU via U(S)ART for STM32 blue pill. At the moment I have no STM32 programming interface for PC. I'm currently able to program the blue pill via Arduino IDE running on RPi.

RPi should be master and blue pill slave. Ultimately, the RPi should be able to pole the blue pill for certain information gathered from sensors connected to the blue pill.

There are a number of modbus slave libraries for Arduino on github.  I haven't used them, but I'd start there...