Dark purple petunias stressed.

A few weeks ago, my petunia leaves were lightening up and started turning yellow. I suspected they were iron deficient due to a combination of alkaline soil and fertilizing. I resolved this issue by adding chelated iron, which rebounded them fast. The leaves have returned to a beautiful dark green and are growing like weeds. Except for the dark purple petunias. They have continued to decline over the past few weeks, the leaves yellowing more, becoming brittle, and turning a purple-black color in areas; they are also no longer blooming. This problem occurs in separate beds and pots. I am really at a loss here. They get the right amount of water, should be getting enough nutrients (they received balanced fertilizer, which I have now backed off on due to the iron issue), have plenty of sun, not too hot here. It’s just super strange that they were all at an even spot, and after adding the iron, the others grew faster than ever while these continued to decline. Any help here would be appreciated. Thanks so much.