They put Blazekin in Uber because of his Ability to Baton-pass Swords-dance/Hone-claws and Speed boosts. If you are thinking: "Well can't Ninjask do that too?" yes but Blazekin can also hit hard and make use of the boosts to his Attack. Personally I believe that he is better suited for OU as Priority kills him, Focus-sash kills him, and several more things kill him. Usually Blazekin has this Moveset: Hi-jump-kick/Swords-dance, Blaze-kick/Swords-dance, Protect, and Baton-pass.
Some good OU counters to him:
Dragonite: Resists both STABs, has Extreme speed/Aqua-jet for priority, and is bulky enough to take multiple hits.
Tentacruel: Resists both STABs, has Haze to remove his Stat ups, has super effective STAB. Gyarados: Resists both STABs, can OHKO Blazekin in one hit from Waterfall.
Some good Uber counters for him:
Giratina: Immune to Fighting, resists fire, has priority Shadow-sneak, Origin form would be the best for KOing him.
Raquaza: Resists both STABs, Extreme-speed priority, hits super Hard.
Honestly most Ubers can take him though, the real problem is beating the Pokemon he Baton-passes too. So it is a good idea to have a Pokemon with Haze or Clear-smog.