thnx
Depends what do you mean by blockchain developer. Implementing the basic principles behind blockchain shouldn't be hard to learn, for instance there is this article from hackernoon which explains pretty well about how you will code blockchain structure with python: https://hackernoon.com/learn-blockchains-by-building-one-117428612f46
If you know HTML, CSS, JS and want to do some front-end development for some blockchain projects you can start to learn things such as SASS, jQuery, AngularJS....
Or you are type of guy who wants to code with existing open blockchain such as NEO, ETHEREUM and others? Then head over there and try to pick free information from documentations, can even ask community members there if you have any questions, find any difficualties.