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... ๐ญ๐ญ๐ญ
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!
In machining, some people have 'rusty hands,' or 'acid hands,' and cause bare metal to corrode faster. Doesn't seem to be a consensus on why. Maybe salty sweat, maybe some particular oil in their skin. It could be that your extended stitching stresses your hands & fingers and encourages them to secrete something that shorter sessions don't. The solution in machining is to wear gloves or to coat everything in oil, cosmoline, or boeshield, but those won't work for metal that is constantly abraded, like needle going through canvas.
You might try hitting your needles with some WD-40 or 3-in-1 oil when you finish a session, or keeping them in an oiled case, but then you've probably got to clean them before use.
Might also be able to rub them down with a high-melting wax, like carnuba or machinable wax to fill in the corrosion pits once they get rough, but needles are probably cheap enough just to throw out when they get too rough to slide easily.