I don’t know if MX blues are the WORST clicky switch out there, but they don’t stand out at all. They’re just so bland. Their tactility is weak, they’re fairly scratchy (like most Cherry switches), and their click sound is satisfactory at best. Maybe these could be made into nice switches with spring modding and lube and slider modifications, but I’d just start off with a better switch if you’re going to put in that kind of effort. I prefer Gateron blue personally, since they’re less scratchy, a bit more tactile, and otherwise of similar quality for less $$$.
Again, probably not the worst clicky switch out there, but probably the most widely-hated by both people who know keyboards and the people who don’t. Let’s call it a win for most popular to hate on.
Also, I’d highly recommend checking some of Chyrosran22’s videos about the worst switches out. They really put Cherry blues in persepctive.
Edit: Chyrosran22 puts Cherry MX Blue on his list of worst sounding switches, but I personally don’t agree with his assessment. I just think they’re boring, not ear-scathingly awful.