I've identified the problem with my black tomatoes. After doing some research I've discovered that it is fusarium wilt.
The plants will start to wilt on one side and very soon the whole plant will be affected. Apparently the bacteria blocks the phloem in the stems. Phloem is the carrier of nutrients through out the plant. The bacterium can stay in the soil for up to six years. Resistant varieties are the only hope when this wilt is present.
Now that I have identified the problem, I realize that it started last year. Three varieties met an early demise in 2011. 'Russian Black', 'Black Sea Man' and 'Burpee Table Talk' had died by early august. This year I again have lost 'Burpee Table Talk' and 'Black Sea Man'. Also, 'Purple Pear' and 'Ivory Pear' have succumbed this year.
It seems that I am going to have pay attention to resistance if I continue to grow tomatoes in the potager. The caveat is that for most heirlooms this information is not listed. There are some OP's that are thriving in the potager. The list includes 'Black Cherry', 'Amy's Apricot', 'Victor', 'Reisenstraube', 'Pritchard' and 'Snowberry'.
I'll have plenty of tomatoes this year despite the losses. I'll just have to do additional research when looking for new varieties to try. Of course, there are always the hybrids...