The Blizzard_Collections is marked as LoadOnDemand (see the Blizzard_Collections.toc file) which means it's not loaded along with the core UI before 3rd party addons (usually loads in response to the player pressing a key or clicking a button).
Your addon will need to listen for the
ADDON_LOADED event and check if "Blizzard_Collections" is the addon and if so, then set up your hooks.