It definitely works to help the plants resist blight. It's not bulletproof, but it's a great cheap method to give your plants some protection.

I use three tablets around the plant when it's transplanted, then another dose every 3 weeks through the season. As the plants grow, I'll increase those additional doses - one tablet the first time, then two tablets for a couple of applications, then three each time for the rest of the season. We have a short season for tomatoes most years, the Texas heat burns them to death by July or August.