The main argument for Calm is not "It avoids the 2HKO from sweeper XYZ", as while there are some Pokemon that close / far from it, that isn't common enough to justify based on those grounds. In stall scenarios, though, Blissey often lacks exactly 100% HP, so making calculations and building a team around just avoiding clean 2HKOs assumes ideal conditions which rarely happen in battles, especially on a stall team. So while a Bold Blissey may take 40% average from a Choice locked move and avoid the 2HKO that way, when you factor in that in real battles you may only be able to keep your Blissey at 90-80% because you took a random Surf, etc, that 40% becomes a lot more threatening, so you want to lower your damage from the attacks you want to wall as much as is realistic and efficient while still maintaining adequate survivability "in a pinch".
For most scenarios, including most offensive or balanced teams, Bold is the better option because Blissey isn't backed up by 5 other walls and may need to deal with Salamence or Scizor in a pinch situation. Also, Blissey comes in less often on these teams, so it's more likely to be at 100% and thus can deal with threats more safely. In stal, battles are drawn out and switches are more frequent, so Blissey is less likely to be at 100% and more likely to take extreme Special hits that it is designed for.
I also think the instant healing of Softboiled facilitates 252/252/4 Bold more, and Wish is more facilitative of 4/252/252 Bold.
