Guroo.com shows the average local cost for 70 common diagnoses and medical tests in most states. That’s the real cost — not “charges” that often get marked down — based on a giant database of what insurance companies actually pay. It won’t reflect costs for particular hospitals or doctors, although officials say that’s coming for some. And it doesn’t have much to say initially about the quality of care. Still, Guroo should shed new light on the country’s opaque, complex and maddening medical bazaar, say consumer advocates. “This has the potential to be a game-changer,” said Katherine Hempstead, who analyzes health insurance for the Robert Wood Johnson Foundation. “It’s good for uninsured people. It’s good for people with high deductibles. It’s good for any person that’s kind of wondering: If I go to see the doctor for such-and-such, what might happen next?” Guroo is produced by the Health Care Cost Institute (HCCI) working with three big insurance companies: UnitedHealthcare, Aetna and Humana, soon to be joined by a fourth, Assurant. The idea is to eventually let members of these plans use a companion site to see how differing provider prices affect their co-payments.”“Buying health care in America is like shopping blindfolded at Macy’s and getting the bill months after you leave the store, economist Uwe Reinhardt likes to say. A tool that went online Wednesday is supposed to give patients a small peek at the products and prices before they open their wallets. Got a sore knee? Having a baby? Need a primary-care doctor? Shopping for an MRI scan?