On the chance it might be viral, could you move it away from any other plants... especially other nightshades like tomatoes, eggplant or other peppers. It didn't show up until you moved it into the cheaper potting mix so that could have an effect. If you have any epsom salts handy, try a couple of tablespoons in the root pouch and water it in sparingly. A couple tablespoons of bone meal probably wouldn't hurt either. It's not quick release like chemical fertilizers so it won't burn the plants but it has the right amount of nitrogen, calcium and phosphate for healthy plants. That's what I used on my chiles this year... that, 2-year-old horse manure and wood ashes. If you use the wood ashes, apply them sparingly... water perking through wood ashes is how they used to get lye for soapmaking.