Thank you for reporting a bug to Unity.
After reviewing the behavior, we've confirmed it aligns with the current design and intended use of the feature. We understand this may differ from your expectations or workflow.
While the CanvasGroup respects its parent CanvasGroup's blockRaycast value, the GraphicRaycaster on the Child Canvas is behaving as expected. When a Canvas has a GrahicRaycaster, Raycasts go down from the Canvas to all of its elements, not up to other Canvases. The Raycasts can pass through transparent elements of child Canvases. CanvasGroups may be used to ignore those Raycasts, but the Canvas with the GraphicRaycaster still receives them. Therefore, parent and Child Canvases which both have GraphicsRaycasters will behave as follows. The child Canvas will handle Raycasts on all if its children, and CanvasGroups can be used to ignore those Raycasts. The parent Canvas handle all the Raycasts on it's children, but not the children of the Child Canvas.
We will close this case as 'As Designed.' If you have feedback on how the feature could better meet your needs, please let us know - we value your input and consider it in future improvements.