The flavanols in dark chocolate can stimulate the endothelium, the lining of arteries, to produce Nitric Oxide (NO), which is a gas (3).
One of the functions of NO is to send signals to the arteries to relax, which lowers resistance to blood flow and therefore reduces blood pressure.
There are many controlled trials showing that cocoa and dark chocolate can improve blood flow and lower blood pressure, but the effects are usually mild (4, 5, 6, 7).
However, there is also one study in people with elevated blood pressure that showed no effect, so take all this with a grain of salt