Blissey is broken in competitive pokemon. Due to it's enormous HP and it's gargantuan special defense Blissey practically walls 90% of special attackers in the game. And those that can somehow deal with Blissey are either running a set that specially tailored to deal with it, I.e Magma storm heatran who's only use is stopping defensively broken pokemon like Blissey and Toxapex. Or are banned, Kyurem being the best example of this, And even then it had to rely on focus blast, Blissey's walling capabilities are so influential that I'd argue that most special attackers that don't have a way to deal with it are completely useless. Let's look at OU's special attackers for a moment here
Blacephelon has trick(A move with limited distribution)
Dragapult(Access to U-turn allowing it to dodge the blissey match up)
Heatran(Forced to run Magma storm+Taunt+Toxic just to beat blissey one on one, Magma storm's 75% accuracy is also unreliable)
Slowking-G(Blissey walls it, forced to switch out)
Tornadus-T(Has to use knock off on the switch and whittle it down overtime with the help of teammates)
Tapu Lele(Has to run Psyshock over psychic which can hurt in some other match ups, because hitting blissey physically is far more valuable.)
Zapdos(Has access to volt switch to dodge the blissey matchup)
Blissey has access to teleport making it an amazing pivot on top of being the best special wall in the game. Has access to rocks and if all of this wasn't enough it even has access to two amazing abilities in natural cure and serene grace. One of which allows you to prevent it from getting whittled down overtime by just switching out.