On the topic of being almost a year late...
C 1201
Given that Obtain Familiar is a feat, I think this could be considered accessible with a feat. Admittedly, Dark Companion is more useful than a familiar, so this feat would be stronger. There's also the level 4 minimum for the feature. I think I would feel safest if this version of Dark Companion had some kind of nerf, like needing an action to command the companion.
A Warlock invocation would just be the feat problem, but worse. I'm also not familiar with Warlock invocations that have long-term durations or permanent costs (and I'd prefer to keep one-off homebrew invocations within those bounds as well). I could see a dark companion invocation coming with a large downside (something that makes you more fragile?) but gives you the same flexible dark companion as a real hexblade.
I will mull this over and try to post something.