I got a Xiaomi GT3 watch for less than 40$ including shipping. it has blood pressure measuring. How accurate? I dont know. I have a normal BP, and it always shows me normal values. Whe I try it on my son or wife's wrist, it always measures a lower BP.
Anyways, I stopped looking at it, and I only use it for the looks and to show me notifications.
I've got a dubious watch bought from AliExpress that was even cheaper and that claims to monitor blood pressure as well as blood glucose levels, blood lipids and even uric acid levels (that's why I bought it as I get gout). It's obviously not going to be calibrated and accurate, and I did get my brother (who's Type 1 diabetic) to compare the watch readings with his own glucose measurement and it was over 10% out, but that could also be a timing issue as he had just recently eaten. Definitely not a device that you would use to determine insulin injections.
Have you considered comparing it to a regular blood pressure monitor in parallel? My guess is that since it isn't certified as a medical device, it's essentially just a random number generator. No smart watch from a big name brand offers this feature yet, so I doubt a $40 device has figured it out.
Anyways, I stopped looking at it, and I only use it for the looks and to show me notifications.