A blended malt whisky is a blend of single malts. There is no grain whisky here, so these are different from standard blended whiskies. These are whiskies made from malt whisky and nothing else, though distilled at several distilleries. In principal, these can have the better features of a number of whiskies, the sum can be greater than the parts.
Compass Box are a firm advocate of the blended malt style. Their Peat Monster features a whisky from the town of Port Askaig on Islay, some whisky from Islay's southern coast, and Ardmore too. Here they've created an approachable, yet peaty dram with lots of character. The sadly defunct Jon, Mark and Robbo have also released some great blended malt whiskies, which have changed the way people think about the category.