Federal environmental officials have called it the nation’s most polluted river, after decades as a receptacle of raw sewage and other toxins. In recent years, the US Environmental Protection Agency has pressed local officials to clean up the Blackstone River, which runs from Worcester to Narragansett Bay. The effort to purge the river has sparked lawsuits and pitted the federal government against Worcester, environmental groups against the EPA, and now Massachusetts against Rhode Island, where state officials are blaming their northern neighbor for fouling their state’s “greatest natural asset.’’