Focusing on anonymity will inevitably bring in the black market crowd, I'm not too concerned it's not like the devs officially support a darknet marketplace or anything. Privacy is far more important of and a bigger issue than drugs will every be though, it's good that ONION fights for it.
you still have to consider that:
private transactions or a big amount of privacy does not have to mean that every transaction is illegal. Just because there is a possibility for high anonymity, it does not mean every transaction is some sort of drug or weapon deal.
This problem sure comes with the higher availability of private transactions with Deep Onion transactions, but it might also be a possibility for anonymous payment methods for countries like china, where everything is regulated a lot.