In past few months, what I have learnt, let me sum it up in short.
First start with fundamentals of blockchain, what it is and how it functions. Now there are several different blockchain but the fundamentals are embedded into Bitcoin blockchain. Thus, one need to have a clear understanding of what and how bitcoin blockchain functions.
Once you get hold of blockchain basics and it's functionality. You may now start reading about some specific blockchain like Ethereum or hyperledger or bitshare to name some. You see different blockchain has different features and API's to access them. Thus, it's like you end up being a React or Angular or Vue.js developer. But all originate from Javascript basically.
For reference:
Do read Bitcoin and Ethereum white paper, they are must.