Not powdery mildew. The leaf itself is actually pale yellow/white. Grown in pot in mixture of compost/peat moss/vermiculite. Have used this mix before growing squash with much success. Plant covered with garden fabric at night to deter pickle worm. Plant in sun approximately 6 hours a day. Plant size about 3 inches tall. Plant age about 30 days. Day temp around 75/80 degrees. Night temp around 55 degrees. No fertilizer or pesticides have been used at this time. 2 other plants in same growing conditions are a little on the pale side, but otherwise normal.
It certainly sounds like you have ideal conditions, but perhaps the young plant is getting too much heat from the sun. https://www.gardeningknowhow.com/plant-problems/environmental/leaf-scald-in-plants.htm
The other likely possibility is that this is a strain that produces white or silvery leaves. If you're sure there's no powdery mildew, I would assume it's just the nature of the type of plant you've chosen.
blistered appearance. what causes this?
You may be describing edema plant disease. The leaves take up water faster than they can transpire it, leaving a blistered appearance. This can be caused by improper watering or fertilization. Here is more: