Here is how I loaded the bootloader on a new STM32 Blue Pill.
Installed Arduino 1.8.5
Tools->Boards->Boards Manager
Install SAM Cortex-M3
Download:
https://github.com/rogerclarkmelbourne/Arduino_STM32/archive/master.zip
Unzip to My Documents->Arduino->Hardware
Rename folder. (Remove -master, so it is simply Arduino_STM32)
Navigate into drivers->win
Execute install_drivers.bat as administrator. (Right-click, run as administrator)
Visit:
https://github.com/rogerclarkmelbourne/STM32duino-bootloader/tree/master/binaries
And select:
Generic_boot20_pc13.bin
Save it in “tools/win/”
Set the Boot0 jumper to 1
Wire up the CH340 USB to TTL-232, G-G, TX-A10, RX-A9
USB (for power)
stm32flash.exe -w generic_boot20_pc13.bin COM4 (whatever com port your CH340 is on)
Put jumper back to 0
Reset
Use Arduino to upload a sketch to test. (Blink, for example)