Canvas on top of another not blocking it - why?

I have two canvases and I’d like to block one with another (ie you cant interact with anything on the other while it is displayed)

One Canvas is on the “Interface” layer, with its interactive elements having a Graphic Raycaster that have a blocking mask set to “Interface”

Over this I display another canvas set to the “Overlay” layer,
it has a fullscreen image with raycast target ON, and a Graphic Raycaster set to Everything, and a Canvas Group whose BlockRaycasts are ON

Both canvases are set to “Screenspace - Camera” and are rendered to two different cameras, the “Interface” layer is rendered to a “Text Camera” and the other to an “Overlay Camera”. The Text Camera is under the Overlay Camera in the stack order on my URP camera and they are both set to their respective volume and culling masks (Overlay on Overlay, Interface on Interface).

This second canvas is meant to make it so that when it displays, the stuff on “Interface” doesnt receive any input, but if I make the fullscreen image half opacity and mouse over buttons on the Interface layer, they get highlighted and can be interacted with.

What am I doing wrong?