I've been working on a side project in the area for a while now. I think what you say with the boxes is pretty sensible and similar to how I see it - all I'll add is that its not just the 'bottom' that needs to be text-based. Some things are just best represented by text - even when you are looking at them in high level or abstracted (especially names of things, numbers, human-use things like addresses, etc.), whilst other things really need no textual representation at all, even if we are familiar with them that way (loops and other control strucutres, as well as graphics [say no to hex-based RGB!], etc.).

My advice is if you want to make it - just start. I farted around for a while thinking 'someone must have done something similar before and have some recommendations', but never really found much helpful (just alot of noisy people pointing to 1970s visual programming systems that failed for reasons that aren't really applicable to a modern project).