Depends on what area. Depending on where you're focusing in the blockchain industry any of the following can be important: cryptography, large scale distribute

Depends on what area. Depending on where you're focusing in the blockchain industry any of the following can be important: cryptography, large scale distributed systems, economics / game theory and designing incentive systems, programming language design, pure functional programming (because smart contracts can't have side effects), frontend development, UI/UX design, and on and on. Ultimately dapp (I hate that term but have long since grudgingly accepted that it's become ubiquitous in the industry) development is just normal web app development except that you might also be making AJAX requests to a blockachin and possibly also writing your own smart contracts. Writing your own smart contracts is a somewhat distinct skillset, but the rest of it is just normal web app development.