I also have this dilemma. Still, this is a weird question, as there are so many other things to learn, not just the recently overhyped topics, being blockchain or machine learning.
I personally believe you will benefit more from learning a backend tool or language, like Node, instead of trying to be proficient in some other area too.
Anyhow it's good to follow the trends, to be up to date.
About blockchain: I would suggest anyone in IT, to investigate the blockchain tech, just to be up to date on it. If you are interested as a backend dev, you should also read about how for example the ethereum VM works, and how to implement smart contracts in Solidity. Personally, I would not put too much effort into it, as I believe blockchain tech is still newborn, and new tools are to come.
About AI: I think in the future you could benefit from being familiar with how to use tensorflow, caffe, scikit-learn or other machine learning frameworks. Half of the globe is looking for big data people, so it can also be a good "next thing to learn".
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