adobe photoshop - Rich black : faded gray appears colored

I have a question about using rich black for a print document. In InDesign, I set a rich black color for the background (0/80/0/100), and I need to set a faded round gray shape, with an opacity around 30-40%, on top of the black background. I create this shape in Photoshop and import the PSD file in InDesign.

Issue : as the rich black contains Magenta, and the gray has transparency on top of it, it appears colored (because of the Magenta in the mix).

Is there a solution to make the gray actually gray on top of my rich black background ?

Thank you very much !