if you look closely at the splash art just above the lantern you'll notice some sort of a head right above the lantern quite close aatrox's head , this does bring up some things since jax's lore isnt exactly clear about his origins
https://puu.sh/wzM0r/5b301928f4.png
So let me put it this way, the version of splash art you are using isn't current to the client and that if Riot intended that there be a connection between the Darkins and Jax directly why would they remove it? Besides that doesn't actually look like Aatrox.
Honestly that "shadow" always looked more like it was just meant to be part of the flames and nothing more.
Also appearantly the darkins are weapons/objects and Jax is supposed to be the guy who can use any weapon equally,which wouldn't make sense if he was bound to one specific weapon.
But jax doesn't even HAVE a real weapon
Jax has already been revealed, if only through mention to be Icathian. Not to mention, he doesn't have the usual appearance characteristic to Darkin.
Reworked Aatrox called jax Vessel. A Scythe was a Vessel for Rhaast so Jax might actually be a darkin
