However, The Jean Genie entered the British Chart on the 9th December 1972 and Blockbuster was released sometime later on the 13th January 1973.  Also the riffs are slightly different with The Jean Genie's riff being "bowmp bowmp bowmp buh-DAH-duh" whereas "Blockbuster"s is "ba-domp ba-domp ba-domp ba-DA-duh".  

The Sweet's writers also deny this.

Interviewer: The Sweet's No. 1 'Blockbuster' had the same riff as David Bowie's 'The Jean Genie' which charted at the same time. Coincidence?

Nicky Chinn: "Oh, absolute coincidence. The ridiculous thing was, of course, they were both on the same record label (RCA). But I know we had never heard Bowie's 'Jean Genie' and to the best of my knowledge he hadn't heard 'Blockbuster'. There was a lot of fuss about it at the time. I think it's interesting to note, and again it was not a conscious thing on our part, but the riff is extremely similar to 'I'm A Man' by The Yardbirds. Fortunately, we went to No. 1 and Bowie went to 2."  

 Record Collector (1998) talking to Nicky Chinn and Mike Chapman