The game already has that system implemented a long time ago. You can check your Inventory > Open Chests > Settings > Auto-Absorb Dark Energy > Gear set check all. When you have that checked, when you use the Open Chests and selected(checked) your Dark Energy Chests in your inventory, they would automatically be used as feeding material to the Black Spirit. I would highly recommend having pets with Black Spirit EXP +% when doing it though, to boost the exp gain of your Black Spirit for additional CP Gain.

Though it is a different case when you manually opened your Dark Energy Chests. You would be needing to use the Open Chests if you want them to be used as feeding materials automatically.