The Blue Lotus really is the first proper Tintin book - a huge step up from Cigars of the Pharaoh. Hergé takes Tintin to the real 1931 Japanese invasion of China, and is firmly and passionately on the side of the Chinese, both versus the Japanese and the Europeans in the Shanghai concession (one of whom in real life would have bee a very young J.G. Ballard). Apparently this came about because a priest who worked with Chinese students at Leuven contacted Hergé out of concern that the promised adventure in China would be as stereotypical as the previous volumes; and through him, Hergé met Zhang Chongren, who was effectively Hergé's co-artist for the Chinese parts of the book, and is also the basis for the character of Chang here and in Tintin in Tibet. Suddenly the political orientation of Tintin has veered very sharply to the left.
But there's also a step change in quality of plotting and of art. There's one rather silly scene where Tintin hospitalises three burly guards, and Thomson and Thompson provide some slapstick comic relief, but otherwise this is a book that takes story-telling seriously and uses the right tools to do it in the right way. It's unfortunate in a way that it ties up some dangling plot strands from Cigars of the Pharaoh, because it is so much better.
I think I actually had not read it before - none of the incidents rang any bells for me, and I see that it was not translated into English until 1983 precisely because it was thought to be too firmly rooted in events of 1931 which would be unknown to today's younger readers. But in fact the themes of military domination and corrupt occupation are, unfortunately, pretty timeless.