My smart watch is a cheap ~$50 Chinese smart watch which has a blood pressure function, but I don't take it seriously at all. Most of the time it reads like 102/62 or something way low like that. I do try to keep the watch at my heart level but it doesn't seem to change much. I did get it to read 121/74 once which I think was probably closer to reality.
It also reads blood oxygen which I think seems to be fairly accurate, usually reads from 96-98%, and heart rate seems to be accurate. But if I really want to check my blood pressure I'll use a blood pressure cuff and not the watch.