EDIT: This is a known issue w/ all pulse oximeters. See Prof Kadambi @UCLA work on this --> https://www.scientificamerican.com/article/fixing-medical-de... FDA is aware of this issue. Increasing diversity in clinical trials is a hard problem. https://www.linkedin.com/events/increasingdiversityinclinica...
And nothing in Apple's advertising or product info suggested this. I also don't think I saw anything describing this in the ToS or EULA. The least they could do is be upfront about this.
My friend is heavily tattooed and their apple watch doesn't even know its on their wrist and they lost alot of functionality because of that. They would love for it to work but understand the limitations of the technology currently.