What I recall was being said, was that whenever our fight or flight mechanism kicks in, the liver generates high levels of cholesterol. So if one is exposed to and reacts to a stressful circumstance then this can lead to higher levels of cholesterol in the body. This was deemed to be of concern to folk who were immersed in stressful situations over significant periods of time. Following on from that, the higher cholesterol levels would then lead to high blood pressure.
The cure here wouldn't be 2 pieces of chocolate daily, but dealing with the root cause of the stress.
If 2 pieces of chocolate daily is going to help reduce blood pressure then I am sure it will be in association with other aspects of healthy eating and healthy lifestyle.