I have only done testing with the STM32F103RB Nucleo board.
I did briefly connected up a generic STM32F103C8T6 ( one with a JTAG programming interface on board) which connected to an external ST-LINK dongle and back to the PC USB.
I had problems B4R compiling/programming this using any of the available generic C8 board types. But, if I selected the "STM32F103RB Mucleo" board type it would compile and program with your BLINK example.
My difference is that I connected my LED to PA5 (being arduino pin 13, not 2)
How are you programming your C8 board?
Which Board Type did you try?
The B4RBoardsExtractor.jar has trouble parsing the board type sub menus available in Arduino. It creates a lot of similar looking Board Types in the drop down list in B4R making choosing the right one difficult.
I'll do some more testing this weekend and see if I can nail down the problem.
You could also try browsing
www.stm32duino.com for any relevant info.
You can try to open the Arduino project created by B4R.
It is located in ..//You-Project/Object/src folder.
You can click on the INO file to open the project in Arduino IDE.
Try compiling it in Arduino IDE.
You can then test different Board Type settings in the Arduino IDE.