I sat back for a day to let the others post advise. I believe it is exactly what they are calling "belt gunk".
The grit on the belt is held in place by being encased in a resin layer. At the too high speed of you 2X42, combined with allowing the blade to get much hotter than when hand grinding,plus pressing too hard,..... it melts and burns this resin.
No real damage to the blade, and probably not much to the belt.
To deal with this there are several things to do.
The best option would be to slow down the grinder....since that isn't going to happen...
The next is to not heat up the blade so much. Start with pressing less hard and cooling the blade more frequently by dunking it in water. Try and avoid long continuous grinds. Grind a pass, lift the blade for a few seconds, grind another pass, dunk, etc.
Things like using a different belt type might help, too, but keeping the blade cooler will be the most simple option for your setup.
The black streaks may be more of a problem with some metals than others because some grind more "gummy".
The black stuff will grind/sand off, so don't worry too much about it anyway.
The final solution to your problem is - Start saving up for a GIB or KMG clone kit. IAAP, make it VS.
When someone says they can't afford a tool or product, a good solution is to lease out your shop.....to yourself.
Put a coffee can with a slot in the lid on the wall or near the door. EVERY time you go out to work on a knife, stick $5 in the slot. You have no problem spending $5 on a beverage, or a hamburger, or gas to drive across town to a store,...so why not save a few for your grinder. In much less than a year you can order the new grinder and have the money painlessly. Epoxy on the lid, to avoid the temptation to "Grab a few bucks".If you raid the can every time you want to go out to eat, it will never get full. If $5 is unrealistic, try $1 and save a little longer.
Another good way to save for a tool is to pay for it in coins. The average person can save several hundred dollars a year just by keeping all the coins they get in change every day. Don't spend any coins. Instead, every night, dump the coins from the day into a big jar/bottle/can. Roll them every six months or yearly and put the rolls in a box labeled GRINDER. Just like the previous savings suggestion...don't spend it on anything but the tool you are saving for.
