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Ob Nixilis isn't in Standard...
Black has no way to deal with artifacts or enchantments; that's been one of its historical weaknesses as part of the space it occupies within the color pie. So you can do one of two things to deal with artifacts - use hand disruption by taking advantage of discard spells like Duress to get rid of them before they hit the board. Or, since you're already splashing red, you should be playing Dragonskull Summit and Blackcleave Cliffs (Blood Crypt after rotation) in the main - otherwise, you have no way to flashback Bump in the Night, and play Rakdos Charm. Yes, splashing red does make your Mutilates slightly weaker, but you're playing so many creatures you probably shouldn't be playing Mutilate to begin with. As a 1-of, it's probably okay, but playing sweepers in an aggro deck is generally not where you want to be.
Also, Sign in Blood is the best black card draw spell in standard right now. You should be playing 3 or 4 of those. Underworld Dreams is another possibility, but since you seem to be going more the aggro route than the control route, that's probably too slow.
Forgot to mention - with only 21 lands and a decently high curve, you need some sort of card draw to make your land drops, and/or you need to go up to 22 or 23 lands. Lastly, always play Tragic Slip before you ever play Bone Splinters in a constructed deck. Bone Splinters is a limited card; it's bad in constructed (yes, even if you have Blood Artist in your deck).