In Gen 3 OU, many notable physical attackers carried attacks like Fire Blast just to hit Skarmory. It's Defense is already really high, so it doesn't need too much investment. It'll also help against stuff like Calm Mind Suicune.
SpDef Skarm is infinitely better because it's really hard to take out with mixed attackers (Fire Blast Snorlax/Flygon, Thunderpunch/HP [Fire] Metagross, etc) and helps phaze out crap like Suicune/Celebi. It's Defense is so naturally high that you don't need to invest.
The same thing happens with Blissey. It's SpDef is already so high that you can easily invest in Def for more walling prowess. Smogon recommends this:
Blissey @ Leftovers
Ability: Natural Cure
EVs: 44 HP / 252 Def / 212 SpA
- Ice Beam / Counter
- Seismic Toss / Fire Blast / Thunderbolt
- Thunder Wave / Toxic / Wish / Heal Bell / Counter / Sing
Soft-Boiled is mandatory because Blissey tends to participate in mid- and slow-paced games and will likely get worn out over the course of a game. Blissey can usually wait to take two or three hits before using Soft-Boiled because of its amazing bulk.
The second move ensures that Dugtrio does not simply remove Blissey with zero effort. Ice Beam hits Gengar and allows Blissey to check mixed and Dragon Dance Salamence somewhat reliably. Counter is good for teams with a Pursuit trapper to get rid of Gengar or for teams that are just a little soft against physical attacks like Metagross's Meteor Mash, Swampert's Earthquake, mixed Tyranitar's Brick Break, and any of Aerodactyl's attacks. Counter should not be the primary means by which these attacks are dealt with because Blissey is usually vital to preserve for mid-game, but late-game, Blissey can lure in and take out physical or mixed attackers just as the opponent thinks a sweep is assured. Pursuit trapper + Forretress teams benefit from Counter, as well as teams that form soft-checking cores between Pokemon like Flygon, Metagross, and offensive Suicune with Zapdos or Moltres to handle physical attackers like Tyranitar and Metagross, with Superman teams—teams that use hovering Pokemon to avoid Spikes—being a prominent example of them. Ice Beam and Counter are not mutually exclusive. Traditionally, Counter, unlike Ice Beam, ensures that Dugtrio gets OHKOed regardless of its bulk. However, the emergence of Beat Up on Dugtrio, which cannot be Countered, puts forth a strong case for Ice Beam to be used alongside Counter.
By doing a fixed amount of damage regardless of the foe's defensive boosts, Seismic Toss is the most consistent attacking move because it allows Blissey to force Suicune to use Rest and beats chipped or statused Calm Mind users that have no ability to recover like offensive Celebi, Jirachi, Suicune, and Raikou. However, it still is not entirely reliable against these offensive Calm Mind users, as Blissey needs to recover quite a bit against them and can get affected by Special Defense drops or paralysis. Seismic Toss also allows Blissey to reliably chip most other Pokemon bar Gengar without concern of the incoming target's elemental resistances. Fire Blast and Thunderbolt relieve the Spikes pressure a bit by hitting Skarmory hard. Thunderbolt also clobbers Cloyster. Fire Blast 2HKOes Metagross and Heracross and OHKOes Breloom, while Thunderbolt puts immense pressure on Milotic, Suicune, Recover Starmie, and Gyarados. It also prevents Suicune from PP stalling with Rest if it is has not yet used Calm Mind, but Blissey should not be used solely to deal with Suicune in any case.
Blissey can run a variety of status moves in its filler slot. Thunder Wave paralyzes most switch-ins such as Tyranitar, Metagross, and Skarmory and helps slow Pokemon like Tyranitar, Swampert, and Metagross outrun them. Thunder Wave is also useful for stopping physical attackers in a pinch. It threatens to thwart Dragon Dance Tyranitar's and Agility Metagross's sweeps and gives Blissey an out against Aerodactyl by relying on full paralysis, all of which, while unreliable, is still notable. Thunder Wave also helps Blissey beat offensive Jirachi. Toxic is a much better status move against Tyranitar not only because it is the only way Blissey can consistently do damage to it, but also because paralysis allows Tyranitar to switch freely into Will-O-Wisp from Gengar. Toxic also forces out Swampert and Blissey and can be useful on Pursuit Tyranitar teams that do not have the physical pressure to otherwise punish opposing Blissey. Ice Beam + Fire Blast + Toxic is a good combination of moves that nails all of Blissey's common switch-ins—Metagross, Skarmory, and Tyranitar—at the expense of being beaten by Calm Mind users. Sing can completely shut down a huge threat like Tyranitar, Metagross, or Skarmory, which can help Pokemon like Jirachi and Aerodactyl. However, if it misses, it can also lure in sleep fodder like Magneton, Claydol, and Pursuit Tyranitar, all of which can be trapped by Dugtrio or one's own Pursuit Tyranitar. Sing, while unreliable, may help to take down tanks like Milotic, Rest Zapdos, and even other Blissey, denying them recovery opportunities as they get chipped down by Seismic Toss and perhaps Spikes damage.
For supportive options, Wish is good for sustaining the team, especially on more offensive Skarmory + Blissey teams. For example, it heals Choice Band Salamence by using Wish as a Fighting-type or Celebi comes in, and it also supports soft Tyranitar checks like Metagross, Flygon, and Suicune as well as soft Metagross checks like Zapdos and Moltres. Wish also supports Skarmory, especially against mixed attackers that hit it hard but can't quite get rid of it and also supports defensive Suicune, which is a worse physical wall than Swampert in sand. Wish + Counter is a common pairing on teams with a Pursuit trapper that helps to minimize the reliance on dedicated physical walls.
Heal Bell supports stall teams that use Wish or Rest on other Pokemon for longevity like Jirachi and Suicune, as well as Snorlax and Vaporeon occasionally, and also allows Claydol to use Explosion over Refresh on teams that have enough support to take down Skarmory. As an auxiliary benefit, Heal Bell also helps Blissey deal with foes that use Toxic like Swampert and Zapdos without having to switch out, possibly take Spikes damage, and perhaps let a teammate take a nasty hit or Toxic again. Note, however, that this can only be done occasionally due to Heal Bell's low PP.
The listed EVs allow Blissey to OHKO bulkless Dugtrio with Ice Beam after one layer of Spikes while giving Blissey maximum physical bulk. If special attacks are not used on Blissey, the Special Attack EVs can go into HP or Speed. Blissey can also use a Modest nature with EVs of 252 Def / 252 SpA / 4 Spe to OHKO bulkless Dugtrio without Spikes and for more damage in general. Note that a non-negligible fraction of Dugtrio carry some special bulk, so considering that Blissey doesn't benefit much more from additional HP or Speed, it's not a bad decision to invest in more Special Attack.
Switch Blissey into special threats or status moves. One of Blissey's biggest advantages early-game is its unrevealed moveset; one can play off the fear of firing off a status move to make a double switch to Skarmory or some other threat. Use Wish as a mid-ground move mid-game when it is unclear whether the opponent will pivot out. The key to winning games with Blissey is to exploit the free turns it gets from soaking special hits, either by forcing chip damage by itself on more defensive teams or by supporting offensive teammates with Wish, aggressive double switching, or exploiting its coverage on offensive teams such that the sequence loops in a way that is unfavorable to the opponent.