Every new technology have both dark and bright side and same goes for blockchain/bitcoin technology.
Blockchain has lots of potential and it can be useful in almost every field from medical to financial. Still many possible use cases of blockchain is yet to be found so we don't know the full potential of this new tech. The only dark side of blockchain is associated with anonymity but at the same time that is also one of the feature which keep blockchain tech above all other old tech.
I think the anonymity and also the immutability could be stumbling blocks. I think in an ideal world bitcoin will be a decentralised digital money of the future in a society that is cashless. My biggest fear is that it becomes a gateway drug or Trojan horse for governments to introduce their own fully controlled and immutable currencies that would leave us all at the mercy of their will.