Many smartwatches (even smartbands) can measure your heart rate, but very few can measure blood pressure. Thats because to measure blood pressure, you need a tight-fitting band that presses into your hand uncomfortably.
A patent shows that Samsung may have figured out a way to make the measurement with a light sensor similar to the heart rate sensors. The patent drawings show a watch, called Samsung Gear X (not to be confused with Gear IconX).
The design shown is similar to the original Gear watches and were pretty sure its just a placeholder (same for the name).
The way it works is it shines polarized light and senses the intensity of the scattered light. By the sound of it, this will still require you to keep the watch tight (smartwatch heart rate monitors dont work if the watch is loose either).
Source (in Korean) | Via