I really thinks it was just sunscald. I know a lot of people were having trouble with basil downy mildew around the forum, but I haven't been so unfortunate here.... Mine just REALLY don't like going in the main bed. Once they start their heartier, tougher leaves, I don't think you'll have any trouble. I wouldnt overreact and pull your plants unless you take a sample to your local extension office and get a more for sure answer.
I can tell you, I plant tender basil every single year, and it ALWAYS does this. I just use it as an excuse to make pesto! Yes!!!
USDA Zone 7b/ Sunset Zone 31