Coworker with domain knowledge not cooperating on resolving a blocker - What to do?

Okay, given that you're working with Bloks, I'm 99% sure I know which Big Tech company you're at 😉. Here's my advice for resolving this issue:

1. Escalate - At the end of the day, everyone's priorities can shift. If Mark Zuckerberg comes by your desk and tells you to do something, you will definitely drop everything you currently have on your plate and do it. Of course, we can't mimic something of that intensity, but you can go for something similar by talking to your manager (who will probably talk to this other engineer's manager). There's a good chance a 30-minute debugging call with this person will resolve your issue and unblock your entire project - I would be surprised if your manager couldn't coordinate this carve out.
2. Widen the search - Every part of the codebase will generally have at least 2 people familiar with it: The writer and the reviewer. Blame the code and ask the reviewers of that module for help. If they can't, ask them for names and continue to traverse the graph. Follow the advice here: "How can I become more independent and better at unblocking myself with tricky technical issues?"
3. Post on Workplace - Make sure to eloquently describe the impact of helping you (i.e. when you get unblocked and are able to complete your awesome project, how does the org benefit?). Meta is a very competitive company, so people are very protective of their time (as you have seen). Make it crystal clear why it's worth it for someone to go through your Workplace question and help you. Here's a good resource to help with that: Bad Question vs. Good Question Example
4. Hack it out on your own - If nobody can save you, do everything you can to save yourself. I recommend this masterclass: [Masterclass] How To Become A Debugging Master And Fix Issues Faster. If you don't have an hour, this video highlight from it is probably the most important: Once You Get This, You'll Be Able To Fix Any Software Bug

The good thing is that all of these methods can be run in parallel. If your manager is down to help, it will take some time for them to get the approval (or rejection). After you make a Workplace post, it will take some time for people to respond. Pinging people that you find from #2 will also take some time as they need time to read the messages and reply back. When it comes to tough problems like these, it's important to take as many bets as you can and hope 1 of them pays off.

Best of luck!