Just an update for anyone like me who is new to cross stitching and were as confused about this as I was lol. Here are a few things I've figured out from tips I got here and things I've researched:
As you can seen the whole needle is black now except for the tip and the eye.
Cleaning did not help. I think it actually made it corrode faster.
As several people have said, once it corrodes it becomes more difficult to use as it doesn't glide at all anymore. It's not terrible with 14 count Aida, but anything smaller would probably be a real pain to use it with.
Gold plate needles will NOT fix this issue. In fact, everything I've read online says they will actually corrode much faster. There are good reasons to pay for gold plated needles, but this is not one of them.
I did find a company online that sells stainless steel non corrosive cross stitch needles, but they cost almost NINE US DOLLARS PER NEEDLE... 😭😭😭
So ultimately it seems if your needle turns black, your best bet is to just keep using it until it becomes difficult to do so, then just throw it out/repurpose it, and use a new one!
You could try replating them with nickel or rhodium at home. Electrolysis is pretty easy to do with minimal costs. You pretty much just need a cheap bench power supply, a Tupperware container, solution, and a piece of the metal you want to put on the needle. All of which you can get easily and cheaply on Amazon.
Probably run you a little more than $100 up front, but it can be done repeatedly before you’d notice the diameter of the needle increasing in any significant way. And of course it could be used to plate other things too. The whole process is pretty neat and it’s handy to be able to do it.
I did consider that, but I really can't afford to spend $100 on a power supply right now.
Another idea would be to lean into them being black and use a bluing agent. $10 or so for a small bottle that will last forever and will help a new needle avoid corrosion for a long time. That’s what they use on guns for the same reason.