Why do beta blockers treat hypertension when beta adrenoreceptors vasodilate blood vessels?

Effect on heart: They reduce heart rate and cardiac output which can reduce BP. Most BP lowering drugs do dilate blood vessels to lower BP and those are generally the best drugs to use for treatment of high BP. Beta blockers are usually add on drugs to reduce tachycardia, arrhythmias and stress on heart that might be helpful to further reduce BP or adverse events especially in patients with heart disease.