When you're asked something more than once, it's good advice to write the answer down. It won't be long someone else will ask the same question. Plus, you won't forget the answers you have last time. With that, here's my take on "How to Become a Blockchain Developer".
I am sorry to say that, like many things in life, there's no definitive answer to this. There are only past experiences and opinions of "qualified?" people who think their advice will be helpful. And also like life, everyone's journey is unique and uniquely joyful. Look around what others are doing, be curious, ask silly questions, go down the rabbit holes
A single course will not make you proficient in anything. These are just credentials. Credentials are just (really bad) proxies/snapshots into your brain for those who are curious "What's inside this brain?". Unfortunately, that's the best we've got.
Still, I will try to draw a rough roadmap for you.
I'd start by getting some familiarity with the subject.
Start bottom up-breadth first: First complete a blockchain course. Get familiar with the fundamentals (see I'm not saying "learn the fundamentals"). This is just to give you a kickstart and get
confused by used to the blockchain terminology. There will be a looot of new words. Don't worry if you don't understand everything. Learning will not happen here.
Remember: Everything only makes sense in context and you need some context.
Next, go top down-depth first: Build something. The easier answer is to build a dApp. Which dApp? Again doesn't matter. You can start small and move on to more complex app. A dApp is not the only thing you can build. You don't have to code. You can write an article, try visualising the Ethereum block, start tweeting, make videos... Get creative. What do you think would be useful for other people learning this? GET YOUR 🙌 ON TO SOMETHING!
Here, the important thing is to go in depth whenever you don't understand anything. Just saw a new word "nonce"? Look it up. 99% of the time someone else had asked the same question. Go through everything. (Try to) leave no ambiguity behind. You might have not understood everything, just come back. You can't imagine how many stupid questions I have to ask in a typical day. Thankfully, only Google knows I'm asking them. You are going to need to use the term in it's context again and that's when you need to revisit and understand better how that thing is interconnected with everything else. THAT'S LEARNING!
The way you get exposed to context, is through hands on work. You are going to need to approach things in many different aspects and see how the concepts fit each time. Work work work... I don't mean work 24/7 but realize it takes time, and be patient.
Lastly, realize there's never an end to learning. You'll never be fully proficient in anything. What matters is learning to learn, to know where to look when you don't know something. Also, learning is only useful when you produce something. You can read all the books in a library. What good does it make if you don't do anything with your knowledge? Can you output things and solve problems with what you know? Good, you "learned" it, call yourself a blockchain developer, if you will.
Now, these pieces of advice are more high level, general, and can be applied to any subject. I will try to get more specific. But before you ask me "which course should I take", let me tell you "I don't know". Because I haven't taken these courses and I can't fully recommend courses I haven't taken. Most likely it makes no difference which one you take. Just pick one and go.
Another question I get often: "Should I learn Solidity?". Usually what actually being asked is "Can I learn Solidity asap and land on a job?". Well, maybe. But it will be really difficult to know what you are doing without having blockchain fundamentals. So I'd say first have an overview of basics.
Nevertheless here are some resources (and resources of resources) I've seen and heard:
At this point anything I add will confuse you more. Just pick and go!