This is quite interesting. As jock says, in the French version of the book the headline in the newspaper actually says "Washington-Tokio: Black-out", and not "Communication broken between Washington and Tokyo" (although they do mean the same thing, technically). I'd also like to know which specific version you were reading, jjtt195. It's certainly not the English printed book anyway.
However, I agree that the postmark date says 15-VII-58, even if it is slightly indistinct.
Another way to date this is by the stamps. Hong Kong was still a British crown colony at the time so they show Queen Elizabeth on the left and her father George VI on the right. A rare example of 'real-life' people appearing in Tintin!
See
here and
here.
Queen Elizabeth's reign began in 1952, but it's usually the case that stamps with the previous monarch are valid for a few years. For example, QEII stamps are still valid in the UK today, three years after her passing.
Either way, it is odd to have a headline that alludes to 1941. I don't believe there was a 'black-out' at any time in the 50s between the US and Japan, but then Tintin isn't actually set in the 'real world'... or is it...?