Hi Duarte Silva. To customize Grbl for the Blue Pill you will have to learn about Atollic Studio. Try this Github site for a start:
That site is based on this one: https://github.com/usbcnc/grbl
Honestly, I moved past the Blue Pill board I was using to the Canaduino board (ESP-32 type) I’ve described elsewhere here. It is easier to program using the Arduino IDE and Bart Dring has done a great job on the software:
This is just a better 32 bit type of board for playing as it also adds wifi and bluetooth for fun. I’ve described the setup elsewhere here. A ‘regular’ type ESP-32 is also suitable. I just preferred the Canaduino type since I can use an Arduino Uno CNC shield with it to drive the stepper motors = simpler for a lightweight CNC machine.
Cheers