I’m on 4.4.4 and have set up 2 ATV2’s with ATV Black and Plex this week just fine (one brand new with 4.4.4 on and one with 4.2.1). For 4.4.4 you will need to be running Plex for ATV 0.9beta4.
I just ran Seas0npass on both (which DL’d latest firmware and flashed it), then install ATVBlack and media player, then I installed Plex manually vis SSH as per the Plex for ATV instructions.
SSH into the apple tv User: root Pass: alpine and run the following lines:
echo "deb http://www.ambertation.de ./downloads/PLEX/" > /etc/apt/sources.list.d/plex.list
apt-get install com.plex.client-plugin=0.0.9.0.0-312c11b0
It is working fine for me except a minor issue where there is a problem with AC3 audio. Known issue, devs working on it for next beta version.