Dooku was a Jedi before he became a Sith, so you could class him as a renegade Jedi....I wouldnt class him as a Dark Jedi though, I'd class a Dark Jedi basically as the same thing as a sith, but without the obsession of trying to take over the entire universe.....(I wouldnt say they used both sets of Dark and Lightside force powers either, that I would call an 'Uber Jedi'
Maybe thats why the Sith are the Jedi's enemy in TPM and AOTC and AATA, because the Jedi are guardians of the Republic, the universe if you will, and Palpatine and his only Sith Friends (Maul, then Dooku, and soon Vader) are trying to take over the universe.....
********************SPOILER BELOW*********************************
If you havent read the book for AOTC's then this might be a little bit of a spoiler for you, but Yoda, Mace Windu, and Jocasta Nu all mention 'The Lost Twenty'. 20 Jedi who have left the Jedi Order, one of them being Count Dooku, surely if Dooku becomes a Sith and there can only be 2 at any one time as a general rule of thumb, then others who turned to the Darkside would be Dark Jedi, but wouldnt be mentioned because they dont have the lust for power that Dooku and Palpatine have......
Thats how I see it anyway...