What is the name of the tune that Black always whistles?
Ep11 seems to suggest that it's Magic Flute's Overture but slowed down.
Well, they did use Ode to Joy as well (Albeit, usage is not as epic as Evan), so I'm assuming that they also selected another classical piece to be in.

In EP 11: 
1659-1723 King of Despair hums that song at an extremely slow pace.
1723-2017 his hum is replaced with orchestra version which seems to increase in speed after a while. 

Compare to 1:27 and set the speed to 0.5. Or a different source at here at 5:38.

credits to ʞɹɐzǝɹ for bringing 'Overture' up.

man you got me thinking about Hibike Euphonium all over again.