Are you supposed to do anything other than stand in the dark to regen hunger as a Shadow? I can dig a hole and stand in complete darkness, and it still won't regenerate it. Is this a possible bug or am I doing something wrong?

To use shadow snack, you need to be in a dark location - the hole you mention should be fine, then press Active Power (Secondary) and you should replenish hunger, with a new bar showing the cooldown. Does this work for you?