Why wouldn’t Dr Doom be able to use his magic from his Marvel Universe against Black Adam despite being in the DC Universe? I can understand Dr Doom not being able manipulate DC Universe magic off the bat but struggle with Doom not using his magic skill set he has already. Plus, I do not think Doom would engage Black Adam without Prep…
With Prep and Magic, I bet Doom would steal Black Adam’s power. Dooms force field/mystical shield can tank celestial shots, Beyonder shots, so Black Adam isn't going to be “one-shotting" Doom, not even in a “speed blitz". So that leaves open Doom's many options. “How about power off my armor and mind switch into Black Adam's body. Maybe I'll even return it to him.” Ovoid mind switch. Doom’s will be supreme. No way Adam “powers” his way out of it. Or alternatively Doom can travel back in time do his research, chill, relax, get his “prep-time on", then pop up a “second later", and really embarrass the King from Kandaq.
Beings with exploitable weaknesses should never fight Doom.