The “Blue Pill” is a cheap STM32 Cortex-M3 based microcontroller board. It is popular because of its price and versatility. Now there is an updated variant, dubbed the “Black Pill” with a new STM32 microcontroller based on the Cortex-M4F (with a FPU).
The Blue Pill uses the STMicro STM32F103C8T6 and costs less than $2.
The new “Black Pill” is shipping in two variants, one using the STM32F401CCU6, the other using STM32F411CEU6. Both are Arm Cortex-M4F based microcontrollers.