The Blue Pill comes without USB bootloader installed. Sometime it's difficult to install the USB bootloader by new users. The serial upload method is easier to use, but it requires to press reset button every time before uploading. There is solution to upload without touching any buttons.
Setting: From Tools manual, make sure to specify "Generic STM32F103C series" for the board type and serial uploading method.
All Blue_Pill_test sketches are written by Roger Schaefer at www.rasmicro.com.
1) You need a 5V FTDI cable that sends out a DTR pulse when uploading, if a cable does not send out a DTR pulse, it will not work. The BPLP depends on a DTR pulse to make auto uploading work. A FT232 or FT231 based FTDI cable will work.
2) You have to use Arduino IDE's internal serial monitor. External serial monitors may not work for the BP-LP.
3) If you need Serial Monitor to debug, open it before uploading is done. When BP is running, opening SM will trig a DTR pulse and it will stop BP from running. When that happens, you need to press the reset button to re-run. That's the reason why I added a big reset button.
4) If you use a full sized breadboard you can install 2 extra 2-pin male headers to support the BP-LP, they should not be installed when a half sized breadboard is used.
5) When a sketch is fully debugged and run in standalone mode, just place a jumper on the BOOT0 and GND, it will run every time after power up.