The bug
24w09a introduces a blur effect to the background when opening the pause menu and using other such menus. However, this blur does not appear to be present when opening the player's inventory or blocks which have an inventory, which seems like a possible oversight.
EDIT: has been brought to my attention that the blog post mentions that this is intended. Any further comments are unneccessary,
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Thank you for pointing this out.
This is intentional:
"Paired with the darkened background is a blur... In-game screens such as containers and books are not affected by these changes"