I do have some thoughts with regards to this suspect. AAA is not my main metagame, so it is very possible that I have some things wrong. Please feel free to correct me.
Blacephalon:
I do not believe that Blacephalon should be banned for 3 reasons:
1. Blacephalon is forced into Magic Guard as it's ability in order to run Mind Blown. This makes its set rather predictable in a metagame where any Pokemon can be sporting pretty much any set, given enough creativity. Any set other than Magic Guard + Mind Blown loses power with regard to Blacephalon's fire stab, making it easier to play around.
2. There are many different ways to play around it. Most Fire, Dragon, Water, and Rock types (if not all), should be able to take a hit from full and then do damage back, with the latter two doing SE damage to Blacephalon. This is also the metagame where Flash Fire is available to any Pokemon, allowing for many different types of checks for Blacephalon to be created, ignoring Blacephalon's weakness to certain forms of priority (Pursuit, Sucker Punch, Aqua Jet, Aerilate Extreme Speed).
3. There are Fire Pokemon can hit even harder than Blacephalon. Victini is the most notable option in my opinion as it is the most free to run different sets to abuse its signature VCreate. Victini can run Tinted Lens or Adaptability with a scarf to hit either hit Pokemon that resist Fire for neutral damage as well with the former, or to increase its damage overall with the latter. Blacephalon does not get that freedom.
252 SpA Blacephalon Mind Blown vs. 4 HP / 0 SpD Zygarde: 143-168 (39.9 - 46.9%) -- guaranteed 3HKO
252 Atk Adaptability Victini V-create vs. 4 HP / 0 Def Zygarde: 139-164 (38.8 - 45.8%) -- guaranteed 3HKO
252 Atk Tinted Lens Victini V-create vs. 4 HP / 0 Def Zygarde: 208-246 (58.1 - 68.7%) -- guaranteed 2HKO
(Keep in mind, Zygarde has 108/121/95 Bulk, and the Adaptability set only hit for 1% less than Blacephalon).
Most Fire type Pokemon have this availability like Victini, like Darmanitan for example.
252 Atk Tinted Lens Darmanitan Flare Blitz vs. 4 HP / 0 Def Zygarde: 176-208 (49.1 - 58.1%) -- 97.7% chance to 2HKO.
Now obviously, Victini and Darmanitan both have their dropbacks, with the former losing its stats each time it uses VCreate with the latter losing health with Flare Blitz, in terms of power, other Pokemon can actually be stronger.
Naganadel:
I will admit, this is slightly theorymonning as I haven't seen too much of Naganadel on the ladder, nor have I particularly worked with it in terms of AAA, so I will keep this section brief. Naganadel has a wonderful speed tier, only naturally being outspeed by base 122's and higher (like Greninja and Mega Alakazam). However, it's typing does leave it slightly vulnerable to -ATE priority as it resists only Galvanized Extreme Speed, and takes super effective damage from Refrigerated ones. Also, like Blacephalon, Naganadel struggles with Flash Fire Steel types. However, unlike Blacephalon, Naganadel is not locked into any specific ability, allowing it to run any of the offensive abilities it chooses, like Adaptability, Tinted Lens, or even Mold Breaker to get past Flash Fire pokemon. Also, Naganadel's movepool is just wide enough for it to get exactly what it needs. It has perfect coverage in Draco Meteor, Fire Blast, and Sludge Wave, with the ability to enhance the damage dealt with a ZMove or an ability of its choosing. Frankly, I am still undecided as to whether or not Naganadel should be banned; however, I am apprehensive of it, and understand why we are having a suspect test on it.