This is an answer to a question posed a few years ago and no longer active.
Black Rock Beach is often mentioned as the only de facto nude beach on Block Island. However, If you want to go nude or topless or wear a thong or whatever, an easier beach to get to is below Mohegan Bluffs near the Southeast Lighthouse. There's a 140+ step staircase that leads almost all the way down. You just have to climb down the last 30 feet or so. Walk to the right over all the rocks until you reach the first promontory and the beach becomes sandy. It's clothing-optional from that point on. Most people wear regular swimsuits, but don't let that deter you. You'll see topless and nudes mixed in. No one cares.
My husband and I go there often. We've walked in g-string and thong swimsuits (me topless) all the way to Black Rock Beach along the shore. It took about 45 minutes each way. We saw several topless women and some nude men and women at various points along the way. If you want privacy, there's plenty of it. But there's no need to get away from the crowds to shed your clothes.Edited: 11:08 am, July 31, 2013