Pancho,
I think what you are seeing isn't decarb. Decarb is a layer of steel that has had the carbon depleted by oxidation ( Thus "de-carb") ,and is softer and non-hardened. Decarb is usually several thousandth of an inch thick, and needs to be ground/sanded off with some effort. If bad, decarb can ruin a blade.
When stainless blades are run through HT, even with a good foil wrap, it often comes out with some light and dark places. Sometimes there are rainbow colors. This is just surface oxide colors, and is not actual decarb.The surface colors are only a few millionth of an inch deep, and with normal finishing this cleans off.
You could try and leave it there, but in my opinion it would not look rustic, but would just look like an unfinished blade. A better method would be to completely finish the blade by sanding to the desired grit, and then applying a patina of some sort to the blade. An etch in muriatic will "age" stainless a lot. You could also try ferric chloride, or a combination of muriatic followed by FC.
This will create an older looking blade with a higher quality appearance.
I have the same problem with folks who beat the snot out of a blade ,leaving deep hammer marks all over the bevels ...right down to the edge ( too much force and too little hammer control). Then they just grind an edge on the blade and call it an antique looking blade. I don't know who the smiths of the past that they revere are, but no smith that was worth his salt would do that poor a job 100-200 years ago.
Better to do a good job in forging and file finish the bevels by hand to a smooth finish. Leave a little "Brute-de-Forge" on the upper surface near the spine, and on the ricasso if desired, and sand to 400 grit. Then etch and patina the blade for an older look.
Stacy
