The chocolate contains chemicals called flavanols.
Flavanols are thought to widen blood vessels, meaning blood pressure is lowered.
The chemicals are found in cocoa products, including dark chocolate.
Some fruit are also rich in flavanols, including apples, pears, grapes and blueberries.
Research has confirmed the link between dark chocolate and a lower risk of cardiovascular problems, including heart disease.
The average blood pressure reduction was about two to three mmHg, a relatively modest amount, according to the NHS.
But, the small difference could have a positive effect on health when combined with regular exercise.
Although it’s high in fat and calories, chocolate can be part of a healthy, balanced diet when eaten in moderation. There is a balancing act between lowering blood pressure by eating the chocolate, and the risk of obesity by eating too much.
High blood pressure – or hypertension – is estimated to affect one in four people in the UK.
In the UK, there are about seven million people living with the condition undiagnosed, according to the British Heart Foundation.
Vital organs are put under pressure with high blood pressure. That includes the brain, heart and kidneys.
An ideal blood pressure is between 90/60mmHg and 120/80mmHg. Making small lifestyle changes will keep blood pressure at a normal level.
The condition increases the risk of other lethal conditions, including heart disease, kidney disease, strokes and vascular dementia.
It’s caused by being overweight, smoking, eating too much salt, not exercising enough, or just being over 65.
Only a blood pressure test will reveal if someone suffers form high blood pressure.
Other foods that help to reduce blood pressure include spinach, skimmed milk, fish, bananas and beetroot.