That notion is a bit of a misconception :-) Yes, soil and roots can heat up in a black plastic pot but it won't really "fry" them. Wholesale growers and nurseries have scores of black pots full of nursery stock out in full sun all day everyday and with minimal loss......if watering is observed properly. This is their sales inventory so if it was significantly damaging to the plants, they wouldn't do it.

You can reduce the amount of heat each individual pot absorbs by clustering together. Or if very young seedlings (which can be not as adaptable to lots of heat and sun), try rigging some sort of shade structure - like a patio umbrella - over the pots during the midafternoon hours.

Thanks gardengal. I knew that wholesale growers did it, but I guess I didn't relate that to being the same thing I am doing.

LOL!! Absolutely no difference. And the wholesalers often have the pots lined out black fabric weed cloth or gravel, both of which absorb additional heat and reflect back.

Full sun is defined as 8 or more hours daily but YOU can determine when your little seedling pots receive this sunlight. Shielding them a bit from the most intense sun at the hottest part of the day won't hurt.