I had the same issue with the black backgrounds still coming up in rendered mode. I went in and ungrouped the node grouping, fixed the color from black to white in the Transparent BDSF node, and then realized I’d have to go do that all over for the rest. So I just created a new node group with the corrected transparency, went through each blade and replaced the faulty node group with the corrected one.
Lesson learned: Watch your transparency colors!