What happens if you die while wielding the magical blastbox outside of daemonheim? Say I purchase one from the rewards trader for 40k dungeoneering tokens, then go to the wilderness with my newly acquired blastbox and get skulled. Will I lose my blastbox if I then die?
Yes. If you are skulled and in the Wilderness, you lose everything. If you are just skulled, but not in the Wilderness, you drop everything in a gravestone, which you then have a limited amount of time to return to in order to retrieve your items.
You can see what you could lose and keep when you die by bringing up the Items Kept on Death screen from the Equipment page. This will show you the items that you automatically keep (some quest items and low level items), as well as the group of items where you will be able to choose three items to keep. Again, any items dropped are stored in a gravestone for a limited amount of time.
You can then click on the What if I entered the Wilderness button to see what would happen if you died in the Wilderness. Note that both of these screens assume you are not skulled.
When you die in the wilderness, if you are skulled, you will drop the blastbox. However, no one will see it and you can run into the wild and pick it back up. Any runes stored in it will remain when you pick it up.
This does not apply to most dungeoneering items. Most items like gravites and necklaces will turn into cash. The blastbox will drop and gem/coal bags will just disappear.