What do you mean by "...planted some Butterfly plants as directed?" There are dozens of plants that attract butterflies, both annuals & perennials, not all of which require the same soil type/nutrients, sun/shade exposure, moisture needs, etc. If you can name the plants you say are turning black, folks on this forum would be better able to offer ideas and possible solutions.

If you are referring to gaura, I have noticed that the plants are often completely green when purchased but when planted in my garden the leaves will become reddish spotted and turn a darker blackish/brown color. It appears to be normal as foilage and plant continues to grow well and blooms. This has happened to both purchased plants and seedlings that I have grown myself.

No idea about sweet potato plants. What variety did you plant? Some of the black ones will fade to a brownish color when in full sun.

a blackening potato plant may some sort of rot happening. What is your watering practise? Do you water too much, too frequently? Whan you transplanted these things, was it during hot summer sun? or in the shade? Did you wet the plants before transplaning? Did you fertilize? If so what kinds of fertlizers? (you are not supposed to fertilize when transplanting)