They probably are just dying back. Its about the right time for early ones to start dying back, mine are.
Late blight tends to attack quickly, ( mid August here) and in moist weather. Within a few days, you start to see dark brown splodges along stems and on leaves. If you do really think it might be blight, cut off the foliage and burn it or dispose of it elsewhere, and leave the spuds in the ground for 2 or 3 weeks before digging up. If you can post pics, that will help. Afraid (or rather, pleased) that I've not seen early blight, so can't advise, but believe it's not as common.
Modified to add picture of my spuds dying back: