Swiss chard

Question

Some of the leaves have turned dark purple. Are they still good to eat?

Answer 1

Swiss Chard can be infected with Cercospora Leaf Spot, though your description does not quite fit.
This sounds like more of a nutrition issue with your plants.
The taste can often be affected in plants that are not green and healthy.
Yes, you can still eat the Swiss Chard.

http://www.extension.umn.edu/garden/diagnose/plant/vegetable/swisschard/leavesspotsorblotches.html
https://www.gardeningknowhow.com/edible/vegetables/swiss-chard/how-to-grow-swiss-chard.htm
https://www.gardeningknowhow.com/edible/vegetables/swiss-chard/harvesting-swiss-chard-plants.htm