silent, quiet, still, dark
默 centers on the absence of sound or movement: being silent, quiet, or still. The 'dark' sense extends this idea to a lack of light, like a quiet, unlit space.
默 combines 黑 (black, dark) with 犬 (dog). The traditional explanation is that a dog barking in the dark is silent or still, but the exact historical development is uncertain.
A black dog (黑 + 犬) sitting quietly in the dark: it makes no sound and stays still, perfectly silent.
For ボク, imagine a silent, dark room where you can only hear the soft 'boku' of a book falling onto a cushion: boku -> ボク.