Bold nature EV's: 148 HP / 252 Def / 104 SpA / 4 Spe
Special Wall
Leftovers Natural Cure Bold
Moveset EVs
~ Ice Beam
~ Seismic Toss
~ Sing / Thunder Wave
~ Softboiled
148 HP / 252 Def / 104 SpA / 4 Spe
The moveset of the "best Special Wall in the game" remains virtually unchanged in the transition from Advance to DP. Even with the advent of Special Fighting-type moves, Blissey is the absolute best option to handle special attackers, sporting a lone weakness to a generally inaccurate Focus Blast. Otherwise, it is simple; you switch into any special attacker, and Seismic Toss or Sing / Thunder Wave accordingly, restoring your HP with Softboiled as needed. Togekiss and Lucario, the only two Pokémon that learn the never-miss Aura Sphere, can beat Blissey soundly without the aforementioned move, which is the only other Special Fighting-type move of note.
I suggest Sing over Thunder Wave for two reasons. First, Thunder Wave is a much bigger liability in DP than it was in Advance thanks to Motor Drive Electivire and, to a somewhat lesser extent, Garchomp and Rhyperior. Second, Sing makes it harder for many of the Pokémon that feel Blissey is mere "set-up fodder" from coming in safely, making Blissey somewhat of a threat which is always a good thing. Sing is also the best and possibly only way Blissey is going to beat or otherwise impair such monumental Calm Mind threats as Slowbro, Suicune (Rest / Sleep Talk variants), Jirachi and, yes, Blissey herself, which I'll get into later. All have recovery moves and aren't even 4HKOed by Seismic Toss (which, by the way, is only available to Blissey in Advance), assuming Leftovers recovery. You should, however, watch out for Azelf when using Sing, if you have activated Sleep Clause already.