No, shadow possesion is not a kekkai genkai. It is merely a jutsu that is passed through the Nara clan.
The Nara clan, Yamanaka clan, and Akamichi clan all have secret jutsu that likely use Yin or Yang (or a mixture of both, still unclear about that for now). It's a jutsu that clans pass through to only clan members meant to give them the same edge that Kekkai Genkai does. If your clan are the only people that know the secret behind it, and they are sworn to secrecy and are loyal to their clan, then you don't have to worry about them teaching the secret of the jutsu to others.
I have yet to see a hidden jutsu like the shadow possesion be as strong as the Sharingan or Byakugan, but it still brings them above your typical "fodder" type shinobi, if only a bit.