This isn't a question, more of an observation.
When I first powered on one of my Digispark Pros, it flashed the onboard LED, as expected. When I plugged in a DigiLED shield, to a pristine Digispark Pro, the DigiLED flashes blue in anti-phase with the onboard LED.

I guess that's the limit of the additional functionality, but it's nice, nevertheless.
It's like a self contained (albeit expensive) flashing LED Christmas decoration!