The way my cardiologist explained it was using a water hose analogy. The tap is the flow of the water and the beta blocker lowers the flow in the sense that the heart doesn't pump as hard. That's why Beta blockers lower your heart rate but Bp medication does not.
BP medication is like the change in pressure when you squeeze the end of the water hose. The more constricted the hose, the higher the pressure. BP medications lessen the constriction.

I'm not medically trained but if your BP was that low without medication, I'm not surprised the beta blocker made you dizzy. Your heart rate dropped as well as lowering the flow. My husband does not have an aneurysm like I do, but he has naturally low BP and takes a beta blocker for an irregular rhythm. He has to take 1/2 dose because it will otherwise make him pass out when he stands up.