Add serial interface to ST-link clone
You can download the binary code from the STLINK V2-1 STM32F103CB with ST Link Utility.

You will need a 128K version (103CBT6) of part so normal 64K Blue pill is not big enough.  I believe the mARM drop-folder transfer utility probably bloats the V2-1 code to requiring 128K.

I have also heard the firmware USB update will not work because of the USB bootloader having flash protection turned on for the V2-1.

You can get the STLINK V2 with SWO function on a 64K BluePill with the ST Link Utility firmware code download from a STLINK firmware updated Chinese clone.  

You can also just cut the 5v runner and take over the connector 5v pins for SWO from uC pin 31 on a Chinese clone.  Need to have a fine tip soldering iron and a steady hand to solder the pin 31 connection wire without shorting to adjacent pins.

With the cost of a Nucleo64 ST32F103 with the STLINK V2-1 at less then $11 (Mouser) you have to think about whether it is worth the work.