A blockchain developer is a programmer that builds applications that interact with the blockchain system. You will be responsible for creating the backend and front-end aspects of the software. Blockchain does not only involve financial transactions, it is also a system used for storing data and performing business transactions such as contracts.
If a company hires a blockchain developer because they use blockchain technology in their operations whether it is to store data, develop contracts, or do money transfers. Your job might also be to modify existing blockchain technology in that organization.