Seedling is black

Stop misting. That's a poor watering habit. Promix comes with a very weak 1ec chargve, give or take. It's a great ratio that won't cause problems with anything. (assuming the HP/BX type bales and not the untreated raw sphagnum peat moss bales that would need a lot of work to make plant ready) If using the raw stuff it's probably too acidic and water repellent. I'm not familiar with their potting mixes as far as nute-charge that comes pre-amended. Water entire volume of substrate - never leaves a dry pocket. Should treat promix like a soilless substrate (unless it's the 'potting' mix), so 10% runoff is necessary once yuo tart fertigating The 1ec charge will last 7-10 days max. So, if you don't add fertilizer early, just get some minima runoff to ensure entire volume is wet. Wait for top layer to start to dry - 1/2 to 1" deep in a regular pot. For a seedling pot, don't let it dry as deeply before irrigating again. Do not mist it constantly. If it doesn't stay wet enough for germination and sprout, use a humidity dome until it sprouts. that way it'll stay moist until a tap root has driven all the way down. No worries about flooding a a plant that hasn't sprouted yet. if this isn't a potting mix, you need a full diet of nutes every irrigation - 1.3 to 1.5 is a generally good spot to start for overall concentration, but the ratio of each nute is important too.

I sometimes don't open for days when doing soil. Coco is different.