I don’t recall any crossovers in Quality Comics, no Plastic Man appearing in Blackhawk or vice versa. If they happened I may have missed them. If anyone knows of any please let me know. Here is the closest thing I know to a crossover from that publisher. It appeared in Hit Comics #32 (1944), when Kid Eternity needed help and called on Plastic Man by shouting out the word “Eternity!”
Plastic Man doesn’t have a lot to do, so crossover might be too strong a word. It is more like a guest appearance. He appears in 6 panels over two pages. When an old lady turns into Merlin the Magician he gives up and zaps back to Police Comics.
I have high regard for the artwork, credited to Alex Kotzky for pencils. No inker known.
Kid Eternity mixes with Amazons. Just click the thumbnail.