USB Device support for STM32F103
Also - R10 on the blue pill - a pull up resistor on D+ may need to be reduced to 1.5 k to work with your USB port. My laptops are ok with 10 k, but my 2006 desktop needs a lower value pull up.
See wiki dot stm32duino dot com/index.php?title=Blue_Pill (hopefully the board doesn't cull that butchered URL.