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Sableye and Crobat can make Blissey's life a living hell, but it's probably gonna switch against them.
Any Fighting type can one shot it with it's physical STAB, but then again, it's probably gonna switch.
Special attackers with Psyshock are good, too, but good players will switch out of your fucking Latios or Alakazam until they are sure you're not running Psyshock.
Tyranitar can deal some decent damage with Pursuit as it switches, can set up rocks, make it's Leftovers useless, and one shot it with Superpower. Probably one of the best counters there are, as you can use an oponent's Blissey to set up
Dugtrio can switch in with ease as well and destroy.
Breloom can switch into anything and set up a sub, and we all know a Breloom behind a sub can be pretty dangerous.
It can also one shot it with it's STAB. Breloom is probably your best bet, as it'll make your oponent think twice before sending Blissey out.