There are blood pressure monitors that take readings from your wrists, so a watch in theory could be able to measure your BP, but to measure your BP generally you need to stand still or the reading might be erroneous. Also, to take a BP reading an iWatch would have to spend a lot of energy, not something you might expect from a product with a tiny battery in it. So I doubt the iWatch will be measuring the BP unless they came up with a much more energy efficient way.
I wonder how much could be tied to the TouchID tech.
It uses RF array to image the fingerprint in a way that sees the underlying structure of the print and even blood flow. I'm imagining the same tech could be used to image the blood flow through your wrist. From there it might take a whole bunch of research, testing and calibration to get meaningful medical information. Even then that might only tell them changes in the pattern more than being able to put hard numbers, still sounds like that would be useful for general health.