How important is blood flow during sex? Must I take a BP pill before or after?

Take as normal. To have good sex you have to good blood flow. That ensures you're getting the right nutrients to your brain, as well as getting the right stimulus to your sex organs. So improving your sex life means avoiding the things that decrease blood flow (nicotine, drugs, sugar, saturated fat, trans fat, syrups, diabetes, high bp), and embracing the things that increase blood flow (exercise, mega-3. As norm.