I think he meant later in the round as the dice would be rerolled. Maybe
Dungeons are not an exception of the rule "gold/black mana can't be used during night/day", but it is a site where is always night. That means you can use black mana (and cannot use gold one), Day Sharpshooting Ranged Attack 2 (during the Day), or Ranged attack 1 (at Night)
gives range attack 1, etc.
I'm also not sure what you mean by
I suspect this means you can potentially turn a black die in the source to some other color to give yourself more options later in the turn
You can't turn a die in the source to other colors unless you have mana draw or something similar. I also don't understand the "give yourself more options later in the turn" part.