Measuring blood pressure (non-invasively) requires applying pressure to the artery and determining at what point that pressure obstructs blood flow. That would be a) challenging to do with a watch, and b) really annoying to the user.
There was a startup that worked on using this, but they failed (due to financial but also regulatory reasons), and then remotely bricked all their sold devices...
I really wish someone would develop this further. Even if it is not as accurate as a normal measurement, the implications would be huge. There are so many people running around with hypertension who have no idea. I also don't see a risk in false positives in this case, since in principle everybody is recommended to have their blood pressure checked - false positives who go to the doctor are then just like people who read an article and go to the doctor, and are weeded out there. False negatives might be a problem - but if you don't advertise it as a blood pressure measurement tool, but just implement it as an additional warning in a smart watch, you'd reduce the false sense of security people would get if it didn't work properly.
https://www.medgadget.com/2012/01/company-claims-optical-blo...