In short, only speed boost blaziken is banned to ubers.
That's wrong. Blaziken as a whole is banned to Ubers.
Is Blaze Blaziken banned from OU?
Relatively speaking, SB Blaziken isn't all that great either, as Zygarde literally walls it out (I've done it 20 or so times on Mega Blaziken), and any +base 101 and above scarfed pokemon can outspeed it, even after a protect.
Also, Omastar. No one uses it, but FS Weak Armor Omastar is evil if all priority users are gone. Talon Flame can't touch it either.
Really what I'm getting at is, is Blaziken banned in it's entirety? That doesn't make much sense if so.
Gastrodon walls Kyogre, Kyogre shouldn't be uber. (Sarcasm)
SB Blaziken IS that great. After 1 turn of setup it can be at +2 Attack and +1 Speed, and each turn it becomes harder to revenge kill outside of priority; if it stays in for even 2 turn, no Choice Scarfer can touch it. Its STAB combination alone has pretty great coverage, hitting the majority of the metagame for at least neutral damage, and after a SD it can 2HKO even things that resist its STAB, making it hard to switch into. It's not to say (Mega) Blaziken has no counters; that's a pretty common misconception. The issue is that if you don't pack specific Pokemon to account for (Mega) Blaziken, it can and will sweep your team with ease. Getting +1 Speed Boost is easy to the point of being trivial. Getting a SD boost, while not a guarantee, is usually pretty easy, especially if you already manage to net a speed boost, since at +1 speed it can beat a lot of its checks. If you're not packing Talonflame or Azumarill, it can't be reliably revenge killed beyond +1. Hence, it limits teambuilding.
Also:
252+ Atk Mega Blaziken High Jump Kick vs. 252 HP / 0 Def Zygarde: 231-273 (55 - 65%) -- guaranteed 2HKO after Leftovers recovery
252+ Atk Mega Blaziken Low Kick (120 BP) vs. 252 HP / 0 Def Zygarde: 213-252 (50.7 - 60%) -- 85.9% chance to 2HKO after Leftovers recovery
That's hardly walling, unless you're trying to use Zygarde with a 252/252+ spread, which has its own issues due to lack of reliable recovery. It means Zygarde can't reliably switch in. Zygarde would be a check, especially considering after an SD it can 1HKO Tank Sets.