Sometimes patients with blocked arteries DO have significant pain in the low back area. When they don't it may be because of the body's natural ability to stay as normal as possible. This is called homeostasis.
Gradual blockage gives the tissues time to adapt to less blood flow. Smaller blood vessels branch out to send blood around the blockage. These are called collateral blood vessels. In the case of obstructed blood vessels, there may be no symptoms or only mild symptoms if the blockage occurs slowly and there's enough time to form collaterals.
Over time you may develop symptoms if the collaterals can't supply enough blood for normal function. The discs will start to break down if they don't get needed nutrients through the blood supply. Your doctor will help guide you with treatment to avoid this problem.