I wouldn't make that assumption at all. Blockly is a scripting language, and like any scripting language its true power comes from what you can integrate it into. Who knows what people have built with it? I've heard of it being used as a way to express filter chains. Blockly itself isn't even involved in the actual runtime execution at that point, and those filter chains are doing realtime processing of audio streams or network packets. In my own project (a game engine), I use Blockly in an event-driven context so that multiple objects can interact with each other; it's not concurrent execution but it IS cooperative multitasking, and theoretically I could get away with running some of those event handlers on different threads (I just don't intend to bother, because it wouldn't gain me anything).
But you're right that at that point it would be 'graphical Go by means of Blockly". Or graphical Python, or graphical C... Blockly on Arduinos is pretty popular!
For the graphical side of things... well, it would probably end up looking like a representation of the textual code. Blockly already allows you to define functions. You could, for example, use a promise-like architecture; define a block that accepts an expression (which could be a function call) and returns a promise, and that block would execute its parameter on a thread and resolve the promise when it finishes. That would provide a basic concurrency primitive, and you could have a block that waits on a set of promises to resolve as a way to pick up the result, or you could use an event-driven architecture where the process triggers a callback when it finishes.
There are a ton of possibilities, and those are just the ones I could come up with off the top of my head.
Now, don't get me wrong, this is no small amount of work. One thing Blockly doesn't have is a concept of local variables (or even of compound data structures aside from lists), so you're going to have to do something to deal with that. And it's up to you to provide whatever shared-data and synchronization tools that you're going to need. But it is entirely POSSIBLE, and it'd probably be a lot of fun to work on.