From my own experience, I have no problems growing Cherokee Purple
and Black Krim here in the Chicago area. One culprit might be your
nighttime temperatures. If they are not at least 55 degrees, the tomatoe
plants may not set blossoms. Here is a web site where someone tried to
correct that problem:

http://gardeneryardener.blogspot.com...ty-tomato.html

Also, I'm finding information that suggests Heirlooms are late to set blossoms,
but I have not found that in my garden.

You can also try growing some of these difficult tomatoes in pots, which you can

move to shady cool places during a heat wave.

Hope this helps,

Sherwin

George wrote:

My Brandywine, Brandy Boy, Big Beef, and Gurney Girl plants are
producing in abundance, but not turning red yet.

However, the Cherokee Purple and Black Krim are not doing as well, with
many blossoms and few setting. It's been well into the 90°s with RH
running between 7% and 44%. The heat has caused some drooping of the
Cherokee Purple leaves, so I give them some extra water, and cover the
cages with old bed sheets during the heat of day.

I'm in metro Denver where we usually have a short season. On Aug 1, I
snip-off the new growth and blossoms that have not set, and hope for a
late frost.