fresh lvl 50 blackmage, what to get first?

hello,
title says it all and i can craft all lvl 50 HQ
please give advice! thanks!;)

Assuming you're excluding tome mat (item level 70) items.

Weapon - Crafted (pretty cheap to make), Wanderer's Palace, or Grand Company... or just farm Ifrit HM for one slightly better than either of those three
Belt - Militia Belt isn't a bad option and not all that tough to make
Accessories - there's a Militia Earring, but you'll likely be using Zircon/Black Pearl stuff for the other slots until you get some dungeon drops/darklight. There's one darklight ring, the other ring that isn't from Coil/Mythology Tomes at the moment drops in AK (chest near a patrolling Dullahan shortly after first boss).

Then upgrade whatever is your lowest item level. The quest AF is enough to do WP/AK/Ifrit with (particularly with an updated weapon). Intelligence increases are king. In very rough numbers you can value one point of int as about 5 points of a secondary stat (a bit inaccurate, but rough ballpark), if not slightly more.

thank you!
what about darklight? should i bought em all? or craft some piece?

Buying your Darklight gear is cheaper (in terms of Philosopher Tomestones) than buying the crafting materials required to make the crafted gear. That being said, there's no reason you can't do one or the other or both.

When I geared my BLM at 50, every choice I made was to get the item that would increase my average item level the most in one step. Admittedly, that was before the reward changes to Wanderer's Palace and Amdapor Keep, which could potentially provide a more optimized upgrade path. Primary stat increases, determination, and critical chance are all good choices.

In the end, as long as you are suitably geared before challenging proportionally harder content, however you choose to do it will be fine.

If you're not rolling in philosophy, or just trying to budget things, don't get DL accessories. Get the accessories from AK. They're only 1 main stat point less than Darklight accessories on BLM. However, no wrist item or second ring drop, so those you can fill with Darklight. You can get the AK stuff while grinding that dungeon for philosophy/myth anyway.

As others said, weapon is always the best upgrade you can get. However, a kind of unspoken but very true tip is... most people will judge you on the upgrades they can see, rather than those they don't see. People will respect someone with a Darklight robe and Lv20 garbage in the accessory slots and legs (things they cant see) but if they see someone who has DL everything but AF or AK robe in only the body slot they'll drop party. Its dumb, but that's people for you. So sometimes it may be more optimal to upgrade a non-displayed slot, but... it'll be easier on you socially to have your best piece of gear be the one that covers the most of your body.

GC weapon is a good grab for BLM 50. You should get Ifrit's weapon if by random chance you get it, but dont sweat it.
More importantly, if you have money, a Vanya Body/hat with a lot of materia is a good way to go.
My opinion: use the GC wand you can get and rush your relic. After that, if you have 900 myths, immediately push for +1 relic.
If you want to hold off on that, use your remaining philo tomes to fill out your jewelry and maybe supplement with a hero ring
or some other cheap myth piece.

Relic and relic +1 are your biggest upgrades and not replaced with anything you can buy, craft or farm otherwise.