You do not need a key to get there, just follow the main plot. You will be able to explore that graveyard after entering the castle, but there is no rush to do so.
However, due to a bug, the key to Nericon's garden opens the graveyard. Besides the one from Blake, there is a second key in the potion seller's place in Ars Magicana. See
here and the
next page about how to get into Walker's private rooms.