EDIT: Also, some rarity loot like Fortine has would be cool. After getting my first AR gun I kinda don't pick up anything else since I don't have a visual indicator of "is this better" and visually the guns are very similar. I also don't want to be exposed for long.
The reason Fortnite has visible rarity tiers for weapons is twofold: it allows the developers to balance weaponry based on its appearance alone (see:
how they balanced an entire class of shotguns), and allows specific weapons that might be overused if they were too common to have weaker, lower-tier versions that underperform slightly (see: just about every gun in the game).
Those two aren't really as important for a mode like Blackout, since a lot of weapons have clear advantages and weaknesses. It's mostly up to you to figure out what you perform best with, rather than what's "the best". It's not like Fortnite where you'll see a gold AR and instantly drop your blue AR for it. The difference between a KN-57 and ICR-7 is substantial, but both weapons serve specific purposes, and it's up to you to decide which one you'll perform better with. Generally, it's better to think about which weapon classes are going to be more appropriate for your match, than which guns are the "rarest". Naturally there's a rarity stat behind the black, but if you see a rifle and you're holding an RK-7 Garrison, the weapon you should be using is hopefully obvious.
A more complex part of the looting meta comes from selecting weapons based on what attachments they can use. The Fast-Extended Mags attachment is super powerful, but only a handful of guns (such as the Cordite, Titan, and Augur) can make use of it. Some weapons (like the Swordfish) can't take optics at all, and others can take a ton of useful attachments, but aren't all that great for most engagements (such as the aforementioned Titan). It wouldn't make sense to rank weapons based on rarity due to this, seeing as some weapons might perform exceptionally well, but can't accept specific attachments players might find useful (such as putting a low-zoom optic on a sniper rifle, which the Outlaw is fickle about).
Only a few weapon types are explicitly colorized by rarity: Gold "Operator" and "Tactical" weapons, which come pre-loaded with a full suite of attachments. Without said attachments, they're no different than any other gun. In my experience with those, it's often better to grab them, dismantle them on the spot, and put their gear on a gun you'll actually use. In regards to non-weapon rarities,
size is often a visual indicator of rarity, rather than color. Bangages > Medkits > Trauma Kits, Lv. 1 armor (flak jacket) > Lv. 2 armor (padded vest) > Lv. 3 armor (padded vest and helmet). That even applies to ammo, where pistol rounds come in small boxes while rifle cartridges are in larger boxes, and heavy calibers are in ammo cans. It's even how chests are organized: dry boxes contain attachments, ordnance, and ammo while red bags contain medical supplies (and occasionally, the extremely desirable "poker chips" item).
