In XY, the base form of Blaziken was quickbanned from OU for the following reasons:
- It was too easy to get to +1 speed by using Protect.
- It was fairly easy to get a Swords Dance opportunity in XY. It could set up Swords Dance on common Pokemon like Tyranitar, Scizor, Ferrothorn and Aegislash that it scared out. After a single Protect, it could scare out Pokemon like Greninja and Gengar that it outsped at +1 and set up Swords Dance.
- Screens support could make this even easier to pull off.
- With a +1 in speed and +2 in attack, it was nearly impossible to stop it from sweeping most of the metagame. It had 120 base power stabs and great offensive typing.
- While it could be revenge killed by Talonflame and Azumarill, you'd usually have to sack a Pokemon to bring them in.
- This led to teambuilding being very restricted, and Blaziken was deemed overcentralizing.
Its ban led to the Mega Form being unusable in OU.
Source 1
Source 2
In ORAS Ubers, Blaziken struggled to find a niche: its damage output often didn't cut it and it was hard to fit on teams. The base form is in C+, meaning it was a viable niche pick. The mega form is in D rank, meaning it was mostly unviable.
In SM Ubers, the Mega form was a viable niche pick in C+, while the base form was D-rank.
You can find more details in my source.