The weirdest situation to me regarding this 'Ultra/SU' discussion is that Beelze BM has never been labeled as Ultra but Gallant CM in the will be despite being in the same mediums and effectively being the same level of power
Blast Mode's situation is kinda odd in general. In some respects, he doesn't seem like a straight upgrade so much as vanilla Beelzebumon seeming like an imperfected form of what he's supposed to be. This rings all the more true when you consider Beelzebumon's
early design, and a lot of the video games at the time had Blast Mode only, no regular Beelzebumon at all. My guess is that he was always intended to have wings, and he was being implemented in a lot of media as such, but somewhere down the road, they decided to relegate it to a Mode Change instead. And within Tamers, Beelzebumon Blast Mode doesn't look all that special compared to all the other Ultimates. Granted, that may just be because D-Reaper was just that dangerous of an enemy (even with Dukemon, it's not like he went on to give a total curbstomping as soon as he went Crimson Mode..... he did do that to Parasimon in the movie, though). You've also got the opening movie to Digimon World 3, and I think some promotional art of the time IIRC, further suggesting the idea of Blast Mode being an equal to Dukemon (NOT Crimson Mode)..... but ironically, Digimon World 3 also required you to level up Dukemon in order to even access Beelzebumon BM (probably doesn't mean much, considering that game doesn't have a standard evolution system anyway).
But then they went and made vanilla Beelzebumon part of the Seven Great Demon Lords, giving him an implied status as one of the strongest Ultimates out there, rather than being a berserker demon that hadn't gotten his wings yet (keep in mind, the main reason he was so dangerous in the anime is because none of the main characters had encountered an Ultimate up to that point...... not counting Jijimon and Babamon, and I dunno if Gulfmon's canon or not). This probably led to Blast Mode being made to look stronger, but I guess they're somehow still stuck on the idea of Blast being a Slide Evolution rather than an explicit upgrade like Crimson Mode, even when his in-game stats say otherwise.