Magnesium CAN lower blood pressure.
After inconsistent research results, the American Heart Association finally came out in 1998 with a study that confirmed the potentially beneficial impact of magnesium on high blood pressure.(1) Too much magnesium can cause serious health effects including severely lowered blood pressure. 
Magnesium Shown to Slightly Lower Blood Pressure
Over a period of eight weeks, 60 study subjects taking magnesium were closely monitored, with their blood pressure measured daily at home and at the office. Although the differences in blood pressure were small when the magnesium-fueled period was compared to the non-supplemented data, it was still noticeable enough to be conclusive.
The Japanese researchers involved in the study found that study subject blood pressure averaged about 2.7 millimeters less of mercury in systolic pressure and 1.5 millimeters of mercury less diastolic pressure. Systolic and diastolic refer to, respectively, the high and low blood pressure measurements separated by the expression "over." And the higher the blood pressure in the patient, the more noticeable the result was during this study. 
Why Magnesium Lowers Blood Pressure
Magnesium is essential for growth and maintenance of the bones. It also maintains nerve and muscle function, keeps the heart rhythm steady, and supports the immune system. Because magnesium is absorbed in the small intestines, it helps to neutralize stomach acids and move stools through the intestines. 
Magnesium plays a role in more than 300 different enzyme systems in the body and is essential to a wide variety of metabolic processes such as synthesis of protein, fat, and nucleid acids, glucose utilization, and muscle contractions.  Research has indicated that magnesium may be effective in managing disorders such as diabetes, cardiovascular disease, and hypertension.
Magnesium Supplements for High Blood Pressure
Though research has indicated that magnesium may play a role in managing blood pressure, you should not replace prescribed hypertension medication in favor of magnesium supplements. In fact, you shouldn't take magnesium supplements without the advice of a physician. Too much magnesium can cause serious health effects. In addition to severely lowered blood pressure, excess magnesium can cause a slowed heart rate, nausea and vomiting, confusion, respiratory paralysis, cardiac arrest, coma, and even death. 
Dietary Magnesium Lowers Blood Pressure
It's not just magnesium supplements that point to normal blood pressure. Those individuals who pursue a healthy intake of fruits and vegetables rich in the mineral are consistently found to benefit from it in this critical body regulation measurement area. Low-fat dairy foods can also have a real impact on blood pressure, when combined with the fruits and vegetables. 
Until recently, the findings associated with such foods were muddied by the fact that these items are also rich in potassium and fiber. However, the gradual sophistication of research and instrumentation tools has finally allowed proponents of DASH (Dietary Approaches to Stop Hypertension) to categorically state that yes, magnesium can lower blood pressure.
The DASH diet is specifically designed by physicians for those with high blood pressure or pre-hypertension symptoms. It was recently rated as America's top diet program by a panel of physicians and other experts and has been shown to impact blood pressure in as little as 14 days.