I disagree, strictly speaking. Regardless of where something was derived, if the finished product has unique qualities when put together it IS unique.
MW and I were talking about the song Set Adrift On Memory Bliss, which samples the Spandau Ballet song 'True' as it's backbeat. It is still unique because the mesh of lyrics with rhythm creates an entirely different effect and feel.
Even though Zeppelin used blues songs on their inaugural effort, what they did with those songs makes it something unique. It's clearly not the same.
Many ideas are not necessarily unique, but when you put it all together the product IS different. Even Elvis covering a song is different because Elvis is singing it.