Dark Side definitely, but by definition - not Sith, Jedi or any group of Forces users. After all, dark side and light side you don't have much choice him. But the rest are just sects, groups, philosophies - If you deny being in one you categorically can't be in one. Kreia was set apart, in her hatred of the Force she developed a new philosophy - unfortunately she didn't have many followers... :P
In my head canon, she is Arren Kae.
Everything slots into place, even that part about the war. After all Master Kavar said 'I thought you'd died in the Mandalorian Wars!
" To which she replies "Die? No. Grew stronger. Yes." Heavily suggesting she did participate.
Personally I think Kreia is not her real name, but an alias in which she hid from the Jedi and perhaps the Sith - if they knew her true identity as Arren Kae AKA Darth Traya - they would kill her on sight. But hey, thats just my opinion.