I interpret the wiki for the beetle as stating it has an onboard LED, "Magic light blue soft BLINK indicator". The project I'm working on prioritizes lightweight overall, so I'd rather not add an LED if I can utilize the one onboard. Anyone know if it is controllable similar to that on the UNO, etc?
Probably, try the manufacturer's web page. But I would try pin 13.
DFRobot Beetle Board: https://www.dfrobot.com/product-1075.html.
It uses a ATmega32U4 and they claim that it is fully compatible with a Leonardo.
It does not install a new build environment and I think it does not use its own board definition. I'm not sure if they use the Leonardo bootloader or their own bootloader.
There is a wiki and a schematic.
The onboard led is at pin 13. That pin has no connection pad, it is just the led. It is a PWM pin, so the led can be dimmed.