I think we also have to acknowledge that it may be obvious to us now that the pictures can stand without explanation, but that is a position that had to be arrived at by experimentation, it wasn’t self evident.
That's a good point. Another example would be Winsor McCay, probably the greatest comic strip pioneer. His earliest Little Nemo strips have under-panel narrative prose, but he soon dropped this device as the series went on, presumably realising (or proving to his editors) that it was unnecessary clutter.
McCay was working in the very early 1900s, and pioneered a lot of the rules, conventions and visual grammar of comics (and animated cartoons), but I think his work had somewhat faded from view by the 1920s, so I'm not sure how aware of his strips Hergé was when starting Tintin. But I think Hergé was very aware and consciously influenced by contemporary American comic strips of the 1920s, and these may have shown him that speech balloons and drawings alone were sufficient and best.
What's odd about E P Jacobs' narration panels, though, is that he started his Blake and Mortimer series in the late 1940s, long after the period when they were thought necessary by comic creators and editors, and when almost everyone else had abandoned the device. Jacob's can't have believed they were necessary; he must just have liked that style of comic-strip storytelling. Maybe he really liked the old-fashioned feel of strips like the aforementioned Prince Valliant and Tarzan.
Or maybe it was his background in opera (he was a professional tenor) that gave Jacobs his taste for stagey formality and over-kill narration. I believe opera audiences are traditionally given a full synopsis of the opera they're about to see in the programme notes, even when the opera is being performed in their own language. Jacobs' strips have a rather formal, stagey feel generally, which he must have wanted, and which is no doubt part of their charm if it happens to appeal to you.
You're right that Rupert the Bear has no speech balloons at all, Tintinrulz. They have brief rhyming couplets beneath each panel to economically tell the story, and then a mass of narrative prose at the foot of the page, which generally simply repeats in more detail what the couplets and pictures have already told you. Sometimes this prose clears up some plot detail that the rhyming couplets couldn't quite make clear, but usually the prose is completely unnecessary and redundant and is best skipped. To be fair to the publishers, at least having the prose at the foot of the page intentionally makes it easy for readers who wish to skip it do so, leaving the pictures and couplets nicely uncluttered, unlike Jacobs' panels.
I believe that Alfred Bestall (not Rupert's creator, but it's best and longest-serving artist) used to make up the stories and the couplets himself, in addition to drawing the pictures. I don't know if he also wrote the foot-of-page prose, though, or if that was added by an editor.