Blood pressure algorithm
First, it may help to understand how blood pressure is measured:

When a cuff is placed on your arm, it is inflated until the doctor or logic in a machine can no longer hear the blood pulsing on the restricted side of your arm. They then watch the gauges and listen as they lower the pressure in the cuff until they hear a more muffled, whooshing heart rate on the restricted side of your arm. Looking at the gauge when this happens shows the pressure in the cuff and and the pressure in your arteries when they are equal. This is the systolic measurement, the larger number.

They then continue to let air out of the cuff until it sounds like blood is circulating unrestricted, and this is the diastolic measurement, the smaller number.

Can this be done with a smart watch? Sure. Can it be done with math? Sure. Can it be done with a smart watch that doesn't exert large amounts of pressure on your wrist? Probably not. There are too many variables from person to person to account for something that is already incredibly variable, like blood pressure, or serum cholesterol levels.

Hope that helps.