Black stain spreading on keris during vinegar cleaning

[apolaki]

I have a really perplexing dilemma. I gave this keris I posted a long time ago a bath recently in diluted vinegar (originally 5% acidity), but I poured and mixed 2+ cups of water.

http://vikingsword.com/vb/showthread.php?t=21446

I let it sit in the bath for a couple days (taking it out to scrub rust off and rinse before resubmerging in the same bath, what happened is black stains began to form on parts of the keris.

The stain can not be scrubbed off. By scrubbing, it only stains the toothbrush and water a dark black like paint. To make things worse, the black stain emits a strong chemical oder I can only describe as similar to burning tires! It is a noxious fume that permeates the entire bathroom with a horrible stench

The exact same thing happened to a moro kris I attempted to clean with diluted vinegar, and it turned out to ruin that moro kris in the end.

Essentially with both the Indonesian keris and Moro kris developed the black stains. Furthermore, the blackened parts of both blades look like they are corroded the blade as though they were burnt in a fire.

Here are some photos of the black staining of the keris that is spreading. What could it be?