well, I'm not familiar with the name of your patron so I don't know if it is from anything religion/anime etc but looking at spells....
Your Storm-fire Blast spell scales like a cantrip but is a first level spell so needs to have a "cast at higher level" mechanic, I'd base it off of Chromatic orb for this with it doing 1d4 Fire & 1d4 lightning damage instead of 1d8.
Patron Spells:
Level 3: Dispel Magic & Counter-spell are already on the Warlock list so you could switch them out for something else, as your homebrew spell does fire and lightning damage you could slip in Fireball and Lightning bolt.
Subclass abilities:
Remember the Cost:
Resistance to Psychic Damage should be alright, at low levels there won't be many spell casters or monsters throwing out psychic damage but I would query why have resistance to psychic damage, if the patron is a psychic power/entity then maybe have the spell list relate to psychic attacks/powers so your homebrew spells does d8 psychic damage per level instead of fire/lightning and maybe switch in Clairvoyance, Hypnotic Pattern or Sending into the level 3 patron spells to keep with a more psychic entity.
The bit about casting a spell needs some clarification, I'd keep it similar to magic initiate, you get a bonus 1st level spell slot that recharges after a long rest, other wise that becomes a free 5th level spell once per long rest from level 9 onward which is a bit much.
Darkness of Betrayal:
You gain darkness as a bonus spell and can cast it once per level without using a spell slot. In principle I like it but I'd change from once per level per long rest to "using this ability you can cast darkness without using a spell slot an amount of times equal to your Charisma modifier (min 1) per Long rest". Darkness is a good spell but not always party friendly and only useful to the Warlock if they have the Devil Sight invocation and I can't think of many instances you would need more than 5 darkness spells.
Windlords Knowledge:
For ease of reference I'd likely change this to gaining either the Skilled feat (for the proficiency option) or Magic Initiate: cleric/druid/Sorcerer.Wizard (for the spell casting option) and give a bonus weapon proficiency in the Windlord's preferred weapon (what ever that may be).
Windlords Wings:
At will flying is pretty useful, I think you would be ok with this if you changed it to an action to poof the wings out and another action retract them instead of a bonus action but maybe also limit their use in a similar way as the magic item: Winged Boots. The DM part of me also wants to know if the wings can be dispelled?
I have been told that this needs balancing. Could someone help me out?
Dark windlord: https://www.dndbeyond.com/subclasses/293419-the-dark-windlord
Stormfire Blast(Goes in the missing tooltip): https://www.dndbeyond.com/spells/294938-stormfire-blast
well, I'm not familiar with the name of your patron so I don't know if it is from anything religion/anime etc but looking at spells....
Your Storm-fire Blast spell scales like a cantrip but is a first level spell so needs to have a "cast at higher level" mechanic, I'd base it off of Chromatic orb for this with it doing 1d4 Fire & 1d4 lightning damage instead of 1d8.
Patron Spells:
Level 3: Dispel Magic & Counter-spell are already on the Warlock list so you could switch them out for something else, as your homebrew spell does fire and lightning damage you could slip in Fireball and Lightning bolt.
Subclass abilities:
Remember the Cost:
Resistance to Psychic Damage should be alright, at low levels there won't be many spell casters or monsters throwing out psychic damage but I would query why have resistance to psychic damage, if the patron is a psychic power/entity then maybe have the spell list relate to psychic attacks/powers so your homebrew spells does d8 psychic damage per level instead of fire/lightning and maybe switch in Clairvoyance, Hypnotic Pattern or Sending into the level 3 patron spells to keep with a more psychic entity.
The bit about casting a spell needs some clarification, I'd keep it similar to magic initiate, you get a bonus 1st level spell slot that recharges after a long rest, other wise that becomes a free 5th level spell once per long rest from level 9 onward which is a bit much.
Darkness of Betrayal:
You gain darkness as a bonus spell and can cast it once per level without using a spell slot. In principle I like it but I'd change from once per level per long rest to "using this ability you can cast darkness without using a spell slot an amount of times equal to your Charisma modifier (min 1) per Long rest". Darkness is a good spell but not always party friendly and only useful to the Warlock if they have the Devil Sight invocation and I can't think of many instances you would need more than 5 darkness spells.
Windlords Knowledge:
For ease of reference I'd likely change this to gaining either the Skilled feat (for the proficiency option) or Magic Initiate: cleric/druid/Sorcerer.Wizard (for the spell casting option) and give a bonus weapon proficiency in the Windlord's preferred weapon (what ever that may be).
Windlords Wings:
At will flying is pretty useful, I think you would be ok with this if you changed it to an action to poof the wings out and another action retract them instead of a bonus action but maybe also limit their use in a similar way as the magic item: Winged Boots. The DM part of me also wants to know if the wings can be dispelled?
The problem is that I only have access to the SRD
Also, the Windlord's Wings feature runs on the same rules as the Dragon Wings feature from the Sorcerous Origin Draconic Bloodline.