I find my tomatoes are happier if I mulch them. Lots of different ways to do this that will work. I use newspapers on top of the soil and then a layer of hay or raked leaves on top of that to keep the papers from blowing away. I use 6 or so layers of paper overlapped to keep weeds down.
Mulching also prevents soil borne diseases from splashing up on the leaves when it rains or you water.
The second thing I do for blight is spray with bleach.
Take 5oz (exactly) of Clorox concentrate (just that one, other bleaches have different concentrations and that might be too much or not enough of other brands) and add that to 1gal of water. Put that in a pump sprayer and, after the sun goes down, thoroughly spray the entire plant making sure to get under the leaves and around the base of the plant. Don’t do this during the day. Make sure there will be at least 3 hrs before the sun will hit the plants or they’ll burn up. Do this every couple days while you have an active outbreak going on and then prophylactically every week or after a heavy rain.
Your plants will look like hammered hell for a couple of weeks but if the vine is still green they’ll come out with a tomato jungle of new growth. First though, all the leaves that were infected will die off. So patience is rewarded. I usually feed them with a tomato food of some sort (I’m partial to Texas Tomato Food right now though).
The fruit production likely halted if your ambient temps went over 90 in the day and 70 or so at night. Tomatoes don’t like to set fruit when it gets really hot. Although the Cherokee Purple should have less of an issue with the heat. Maybe they’re unhappy for another reason and somebody else will have a good idea about that...
Hi - I was certain that I posted a reply to you m-d. Here I go again! :) We had a call two weeks ago that was almost exactly like yours. A lady’s tomato plants had leaves on the bottom of the plant turning brown and dying. The problems moves upwards. The agent on duty recommended Daconil and copper fungicide in order to treat Septoria Leaf Spot. He said that this will slow down the destruction of the tomatoes she had left so that she could still harvest some fruit. Here is a link to a site that includes a foliar fungicide spray schedule that you might want to consider using next year to stave off another bout with this disease. It also has photos to compare with what you have. http://tomatoes.ces.ncsu.edu/tomatoes-page_id2118/
I hope this is useful!