In SS Singles, there is absolutely no reason to run a mixed Blaziken set. It is best to either run completely Physical or Special Sweeper sets for Blaziken. With having great Special Attack and Attack Stats plus Speed Boost and amazing coverage, it is best to utilize that On only one side of the spectrum to make Blaziken hit as hard as possible. Due to this, and all of Smogon's sets that are Sweepers run fully Physical or Special sets, it's best to do fully Special or Physical sets.
However, there are niche scenarios where you could run mixed. These mostly involve being able to lure in some of Blaziken's checks (e.g. Hippowdon) and hitting them with a move like Overheat, OHKOing them.
So, there are some situations where Mixed Blaziken can be somewhat viable, but running fully Special or Physical is much more reliable and is the option that I prefer.