The term blocking isn't well defined. How slow does an operation have to be to be considered blocking? There are no rules. Fetching google.com/robots.txt is by universal agreement a blocking operation. Reading a file from the filesystem is considered blocking in JS but may not be in other languages, especially given that very fast disks like Optane or modern NVMe Flash can be only an order of magnitude or two slower than RAM. Taking a lock may or may not be considered blocking. Computing pi to a trillion digits would usually not be considered blocking because it doesn't make any async OS API calls, even though it might cause long delays before it returns, but if the author chose to implement it as an async iterator then it could be considered blocking.
Worse, async vs not async is about latency requirements of the caller, but the function author can't know that. If you're in a UI thread doing 60fps animation maybe even reading a file would be too slow, but if you're in a script then it doesn't matter at all and you'd rather have the simpler code. You're calling the same function, the only thing that changed is the performance requirements of the user.
So "potential to block" is to some extent a makework problem that occurs in JS because V8 isn't thread safe, and V8 isn't thread safe because JS isn't, because it was designed for the web which is a kind of UI toolkit. Such toolkits are almost never thread safe because it's too much hassle to program a thread safe renderer and would make it too slow, so that browser-specific implementation constraint propagates all over the JS ecosystem.
Final major problem: some languages use async hype as an excuse to not properly support threads at all, but that's often the only way to fully exploit the capabilities of multicore hardware. If your language only does async you'll always be restricted to a single core and you have to hope your use case parallelizes embarrassingly well at the top level.
If you have threads then this whole problem just goes away. That's the core argument that Java makes with Project Loom. Instead of introducing colored functions everywhere with poorly defined and inconsistent rules over which color to use, just make threads work better. Suddenly the binary async/not async distinction becomes irrelevant.
Still, there's a good reason most languages don't do the Loom approach. It's a massive PITA to implement. The Java guys are doing it because of their heavy investment in advanced VMs, and because the Java ecosystem only rarely calls out to native code. It'd be a lot harder in an ecosystem that had less of a focus on "purity".