Single-malt scotch is the product of one distillery, while a double-malt scotch is a blend of two or more distilleries. Any scotch can come from multiple batches or barrels, but being classified as single-malt simply means it was sourced from one location.
Since a single-malt scotch can be a mixture of multiple barrels from the same distillery, almost all scotch is a form of blended whisky. The formal way to make a blended whisky is to mix scotch from both a barrel-aged whisky and a grain whisky. However, it can also be made when any two types of whisky are blended together and sold as one.