[Theory]
Since Indura transformation is dependent on power lvls, I'm just going to assume that certain lvl Super Demons have mastered such a form, bending it to their will instead of being subjected to being mindless beasts.
[Analysis]
We know drinking Demon blood can affect behaviours. The HKs drank red Demon blood & underwent a metamorphosis. Those that were weak perished, but those that survived gained power & became very aggressive in nature—just like Demons without at least a PL of 50k would eventually perish using Indura, but those with 50+k would survive & gain power but behave like very aggressive beasts.
Had the HKs drank a grey Demon blood, most would've perished, but those that survived would become even more aggressive (like Grey Hendrickson).
So I think it's safe to assume that the factors which influence the rate of a successful Demon transformation includes; determination (maybe Spirit stat), power levels and (possibly) aura ominousity.
[Test Subjects]
- Galan can drink Red & Grey Demons all day and he won't add anything to himself except maybe weight and size. His PL won't chance as him drinking greys & reds is tantamount to a man used to drinking very strong liquor suddenly having light wine hoping he'd get drunk. However, if Galan drank something like an Indura's blood, he's likely to get intoxicated, pass out (maybe die). This is due to Indura's massive PL & potent ominous aura, making their blood very toxic indeed.
-Monspiet can handle Indura as he has the prerequisite; however, when he utilises it, the beast within takes ahold of his mind.
- Estarossa can handle Indura, & probably much better than Monspiet because his aura, spirit & PL are superior to Monspiet.
-Demon KIng & Meliodas (prime) bend Indura to their will, thus making them the Supreme Demons of Darkness. It's also possible that they can remain in a state of Indura always as they've become accustomed to that lvl of Darkness
[Extra]
Indura mode is described as a state in which a superior Demon (50+k PL) would form a contract (or something like that) with the Darkness, consequently revealing their more primal instincts.
Therefore, the more a Demon becomes accustomed to the Darkness, the more Darkness he/she could absorb & command. The 10C always leaving their Demon mark on is one means to become accustomed to the Darkness & grow—they'd become stronger & their auras more potent. Zeldris' (& maybe Estarossa's) mark might undergo a metamorphosis, becoming something like current Meliodas' mark. Eventually, he might be able to bend Indura to his will and become a Supreme Demon of Darkness.