HI @freynder ,
I just realize that when you speak about SERIAL_USB you really speak about.....Serial through USB connector 
.
I believe you speak about Virtual Com Port through USB in ST-LINK.
So I had change R10 on my blue pill and install maple driver.
Now I have a Maple Serial COM mounted when I connect my blue pill with the USB connector .
And......................TADA.................It is working. I had output on my terminal.
Thanks again
