There isn't a single mentioning that angels can't learn black magic, Raphael simply says it's forbidden for them (Azazel who created it was an angel, MC despite lacking wings is also one)...but why would they give a single fuck about that when they already defy god ? Azel even proudly states she doesn't fear him.
Also
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Why not go herself, i mean that's kinda the obvious choice, no ?
Emma has nothing to fear from them, Azel could massacre them all and i guess Raphael too...Azel even told them where she droped off the MC...
Or even better go together, that would also a good approach to redeem the family.