Here in the U.S. there are three main "varieties" of blight... early, late and Septoria. I've experienced two of these, early and late. Although the plant is greatly affected, the fruit that has set will continue to develop and not be affected by the affliction.
Did you plant tomatoes in the same space you used last year or on ground used for potatoes? This is one of the primary causes of the fungus induced blights. Humid weather can trigger an infection, but generally it's due to splashing rain or overhead watering causing the fungal spores to splash up on the lower leaves and make its way up the plant. Drip watering of the roots is best. You could also try a product with Neem oil... I use a brand available here called Green Light. Neem oil is environmentally friendly and I've had good luck with it... Best of luck!