Do you want to work on a project or platform that is already established, or do you want to roll your own blockchain?
I assume you'd want the former, so in that case head on over to github and find the repositories for your favorite cryptocurrencies and go from there, using the languages they're using.
With Bitcoin, this is C++ at the contributor level, or bitcoin script at the higher level. With RSK (rootstock), the platform being built for smart contracts on bitcoin, this is apparently solidity? I haven't kept up, but I guess that's the route they went down to ease transfer of developer knowledge in order to convert ethereum developers.
For Lisk, I believe they just use plain old javascript.
For Cardano, I think they use a special language similar to haskell, called Plutus? The focus of Cardano is on provable security, because one of the greatest weaknesses right now in the ICO space are insecure smart contracts. Bitcoin solves this by making it so that bitcoin script isn't Turing Complete, while Cardano wants to have more of what Ethereum has.
I think the most potential right now is with either Bitcoin or Cardano: