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!
Steel is often treated with black oxide finish on tools and objects that are to be handled. Drill bits, guns, lots of stuff. You could probably rub the item with kerosene or mineral oil if it's rubbing on fabric in a way you don't like. Industrial looms use these to lubricate machines so thread flies along where it needs to go, without snags. Might be able to use parafin wax or BLO/beeswax for a longer lasting lubricating finish.
But yeah, as I said before (I think it was in response to your earlier thread) it's just from handling. Won't come off on fabric or anything. It's part of the metal now. Larger metal pieces could be polished on a wire wheel or grinding wheel, but there's not much metal here. Best to just keep using it while you can. Maybe coat it with a very mild clear lubricant/sealer.