Solving the Bluemont Manor Strongboxes in Crimson Desert is arguably a little easier than the Hernand Castle Strongbox. After all, these don't require you to replicate a melody, but instead you have to mess around with an owl picture and some strange pistons. Though you won't need to get a key to access the room with the strongbox, you will need a mask to steal the items inside at the end.
I can't confirm this, but to gain access to Bluemont Manor, you might also have to complete the first two stages of the First Trial of Trust quest that unlocks in Chapter 2, and is part of the Estate in Dismay faction quest for House Roberts. All that said, here's how to open the Bluemont Manor Strongbox.
