I don't know if there is any future for blockchain or not, but there might be and I am currently a bit bored of what I've been working with so far, so I thought it might be interesting to work with something as new and undeveloped as blockchain just to get that "hacker" feeling back, which I had when I started with web programming back then.
UPD: could you keep your sarcasm to yourself, please? It's not a place to discuss how much of a bubble everything blockchain-related is.
You should have the skill to always be asking:
1. Could this business be built without blockchain as a technology? (suitability of the technology for the use case)
2. Could this business succeed without blockchain as a feature? (hype filter)
Then beyond that, you need tremendous CS fundamentals, as well as pretty deep understanding of cryptography, distributed computing, networking, databases, application security, network security, information security.
You should also ask why you want to work at a blockchain startup.