Have you done any of the quests which give you an invitation to the castle? That should be enough to have Blake offer you the key, despite his mention of reputation.
In the download version or the updated disk version (CD version
patch) you can open the Nericon garden gate with level 5 Lockpick. You can get 2 levels of Lockpick by joining the thieves guild in Verdistis, though that results in a small reputation drop. If you have not done so already, you need to join if you want to complete the holy item quest. One level of Lockpick is gotten
There is another key in the potion seller's back room in Ars Magicana. For a description on how to get to it, see
Stealing from Alchemist Walker and
part 2.
Alternately, you can get past the gate or get a replacement key;
Teleporting Anywhere, via hex editoror
Lost Items - guide and directory of inventory files
