Blights are bacterial diseases that infect the plant through soil. Mary, it's very early for you to be seeing any blight in those small plants! Did you buy them already started or start your own? I'm thinking the soil may have been infected when you got them. If you started with sterile potting soil, and grew your own, well that's amazingly quick for them to be infected.
If you bought transplants, and are thinking to buy more, try this first (or treat new ones as soon as you get them home) - douse the pot with a Hydrogen peroxide solution once a day for a couple of days. You can buy a bottle of H2O2 at the drugstore for a couple of bucks, and you dilute it about 32/1 before treating.
It's a hit-or-miss thing but sometimes the plant still dies and sometimes it recovers! Worth a try. It almost never works on a big, established plant that gets hit by blight. Just too much plant tissue affected all at once and by the time you see it, it's too late. But I've used it on other plants that get hit by sudden blights, like some basils last year, and it worked wonderfully.
