Embedded software from JeeLabs
Embello Home Page
Hardware Documentation
Forth Library Documentation
These boards have been available for quite some time from eBay and AliExpress. Just search for “STM32F103C8T6 board” on those sites.
(this diagram is from an excellent page about the Blue Pill on STM32duino)
The LED is on PC13, “0” is on.
Schematic: as PDF (copied from the same STM32duino wiki as the diagram).