A further refinement is to inherit the arm//stm32f4 BSP to create a Black Pill one:
[arm/blackpill]
INHERIT = arm/stm32f4
BUILD_TESTS = False
STM32F4_ENABLE_USART_2 = 1
STM32F4_ENABLE_USART_3 = 0
STM32F4_HSE_OSCILLATOR = 25000000
I use this in RTEMS Deployment for the AMD K26.