Noah knew that the Dark blast spell wouldn't do enough damage if Adrian managed to use the humming sound again, so he had to make it detonate when his uncle wasn't ready.
The only way to do so though was to make it explode right after a clash, meaning that he wouldn't be able to escape its area of effect.
However, why would he care about his own attack when he didn't fear the spells of solid stage cultivators?
Noah had hidden the enlarging sphere behind his figure during the battle, waiting for it to become threatening enough.
Then, when almost half of the "Breath" in his dantian and of the mental energy inside his mind was poured inside the spell, he decided to make it detonate.
Zac enveloped his figure as his body became ethereal, Noah couldn't use the Black hole spell to absorb his own energies, but those defensive methods were more than enough to block the blast.
