They appear black because they do not rreflect light but rather than absorb photons as heat they absorb them as electricity. This conversion means they do not get hot like a painted black surface does. In fact, solar panels are heat sensitive and become inefficient if too hot, so some have cooling on the back side or even water cooling.

And a much smaller footprint. It could even be binary data for tweaking your algorithmic profile, say the name of a branded product or in the case of a product with few options just the type of item. Audio runs in the megabytes per minute, transcripts in the kilobytes, but reducing to a conclusion of interest in a single specific item is really very small, hard to notice tbh.

Orgzly is my favourite note app. I use that and sync to a folder which I can then sync with Syncthing to my desktop and have versioning there.