I'm thinking of making the spell a level 1 copy of either fog cloud, but with darkness rather than fog. Or darkness, but with non magical darkness. The purpose of this spell would be to use with gloom stalkers umbral sight. Would this be balanced?
No. If it's created by a spell, it's magical by default.
Simply put, if something is created by a spell, it's magical, by the definition of what constitutes a magical effect in this game. This includes the darkness created by spells like darkness. As such, a spell creating non-magical darkness is a contradiction in terms.
However, as pointed out in the comments, it might be plausible for a spell to "create" non-magical darkness by extinguishing light sources in an otherwise-darkened area; this can be accomplished with the cantrip control flames, or the 2nd-level spell pyrotechnics (which also causes additional effects as the flame is extinguished), though in each case they're only able to extinguish flames in a 5-foot cube. Homebrewing a higher-level spell that accomplishes this effect in a larger area but with less flexibility seems unlikely to be broken.
It should be noted that since control flames is a Druid cantrip, it is accessible to a Ranger with the Druidic Warrior Fighting Style.
Under normal circumstances (as in, without Umbral Sight involved), a 1st level spell that works exactly like Fog Cloud except that it creates nonmagical darkness instead of fog might possibly be balanced, maybe, but probably isn't (and probably isn't worth making either way, since it would probably just make the game less enjoyable for everyone).
Depending on who involved in the fight has Darkvision, it could work exactly like Fog Cloud, or it might do nothing, or it could grant most/all of the group advantage on all their attacks and grant most/all of the enemies disadvantage on all of their attacks. Adding Umbral Sight to the mix basically just extends the best case scenario to that character basically all the time (making it overpowered more often, rather than hitting greater heights of overpoweredness).
Now, since the spell creates nonmagical darkness, it would be easier to counter than Fog Cloud since any torch, Light or Dancing Lights spell, or whatever would get rid of it, but that requires that enemies either already have those ready (in which case whoever would potentially cast this spell would presumably know and just not waste their time), or requires that one or more enemies spend actions getting those going. They also need to have those available to them in the first place, and know to use them (which they may or may not, depending on how much they know about magic and how common this spell is in that setting).
So the spell is potentially far, far more powerful than even Darkness, which is a second level spell, but on the other hand it can be countered easily by competent opponents. I don't know what level I'd give this hypothetical spell overall, but I'd say it's probably not worth making since so much of the time it's either going to be way too powerful or way too weak.
If you really, really want to create a spell that creates nonmagical darkness so that character can take advantage of Umbral Sight, maybe think about something (with a smaller radius, maybe 10ft) that will last until the end of the caster's next turn. That way the Gloom Stalker can cast it one round, get some defensive bonuses until their next turn, then attack with advantage. It's powerful under the right circumstances, can still be countered easily by knowledgeable enemies, but unless the Gloom Stalker spams it continuously it might not be worth taking the time to counter.
If we compare it to Shield, which is a first level spell, it's stronger in some ways (since it gives offense and defense) but also weaker in others (since it would take an action to cast, not a reaction), plus there can sometimes be benefits to giving people disadvantage on their attacks (Rogues cannot Sneak Attack when they attack with disadvantage, for example). That might possibly be appropriate for a first level spell, but I'm leaning more toward it being second level. That said, if you make it second level you might want to let it last an extra round for each additional spell level it's given.