It's not sampled. Plus you can't copyright, groove, chords and the general feeling of the sound. Do I believe that Pharrell ripped off Marvin Gaye on this song? Yes. But is anything copyrightable, like melody or words infringed upon? No.
There's no question that "Blurred Lines" has the same sounds and feel as Marvin Gaye's song. But it would be like the millions of Nirvana copycats that came out sounding like the band in the early 90's. Just because it sounds the same, doesn't mean that it is the same.