"Blooming" is a genuinely creative choice, not a literal one. 盛 (sei) carries no floral sense at all — it's the character in 盛る (to heap up, to be at its height) and 繁盛 (thriving business), most often paired with fire to mean a blaze reaching full strength, not a flower opening. VIZ reaches for garden imagery anyway, likely because "flame at its peak, undulating" reads clunkier as a combat callout than "blooming flame undulation" does, and because "blooming" carries its own connotation of something expanding outward — which happens to match the wide, spreading arc of the attack on the page reasonably well, even if it isn't what the kanji themselves say.
The corpus's OCR pass only partially captures this callout, rendering it as the truncated fragment "肆プ型〝盛炎のう" rather than the full 肆ノ型〝盛炎のうねり〟visible on close inspection of the page — one more instance of technique-name lettering (set in a distinct display font from ordinary dialogue) defeating automated character recognition, a recurring theme across this dictionary regardless of breathing style. This is also, notably, the only occurrence of 盛炎のうねり anywhere in this corpus: a single blow, thrown once, in the handful of remaining pages before Kyojuro Rengoku's death — one of several late-numbered Flame Breathing forms the reader meets and loses within the same short stretch of Volume 8.
The technique also answers a specific tactical problem the manga sets up one panel earlier: Akaza's punch travels through empty air to strike Rengoku from a distance, and Rengoku's own internal narration immediately flags the resulting standoff — fighting at range makes it "tough to cut off his head," the only way to kill a demon. Blooming Flame Undulation's broad sweep reads as an attempt to answer a long-range opponent with a long-range attack of his own, before the fight resets back to close-quarters exchanges for the rest of the duel.