Skymin's Choice Specs Seed Flare does 215-253 (234.33 average) damge to 307 SpcDef (standard - Bold and no SpcDef Evs) Blissey. If Seed Flare's effect works, she now takes 429-505 (467.39 average) damage. This means if she swaps into this attack, receives Seed Flare's effect (which, counting accuracy, has a 68% chance of occuring - its likely), and then stays to be hit by Seed Flare again, she takes 644-758 (701.72 average) damage. If SR is in play, this means that the standard Blissey (714 Hlth, 307 SpcDef), is very likely to be 2HKOed by Timid Choice Specs Seed Flare.
Blisseys will have to start investing in SpcDef if they want to be a true Skymin counter. Furthermore, Blisseys, with no SpcAtt EVss (and bold) can't OHKO Skymin with Ice Beam unless SR is in play.
If Skymin is a subseeder, and has 252 Hlth, Blissey's 252 SpcAtt EVs (and Bold) still has less than 50% chance of OHKOing Skymin (it does 370-435 (402.88 average) damage). This means that Skymin can set up leech seed on Blissey and stall to death with Substitute. If she swaps out, Skymin gets a free hit on the next pokemon.
So, in short, Blissey, the most famous SpcWall and arguably the best, is not a reliable Skymin counter. And even in other scenarios, Skymin can also launch a Air Slash with a 57% chance (counting accuracy) of flinching Blissey.
Skymin alone has massively increased Registeel, Regice and Timid Scarftran numbers, as well as more SpcDef focus in Metagross and Skarmory and promoting Mamoswine numbers. She is severly affecting the metagame, and has very few actual counters. Garchomp was banned for these two reasons (due to his Yache Berry, though Skymin doesnt even need the berry due to the chance factors of Seed Flare and Air Slash making many other pokemon unreliable counters); Skymin should be next, other we will have a heavily chance-based metagame.