Are there any Pokemon in Gen 4 and 5, legendary or not, come close to challenging Blissey's defensive prowess?

Yes

No way

No, no Pokemon has the combined stamina/defense that makes Blissey so strong defensively

Garchomp is expected to outdo Rayquaza, Dialga is expected to outdo basically everything/everyone. Idk about Infernape how his stats/moves will translate. Conklederp might be interesting too, could be another Hariyama (I expect Conkle will actually be insane).

Pokemon with the highest tankiness (def*stm)

1. Blissey
2. Zygarde, complete forme (un-nerfed)
3. Giratina, altered forme
4. Chansey
5. Lugia
6. Ho-Oh (un-nerfed)
7. Cresselia
8. Arceus (un-nerfed)
9. Primal Kyogre/Primal Groudon (super un-nerfed)
11. Giratina, origin form (un-nerfed)
12. Snorlax

Blissey will forever be the best defender because of it's solid defense and ridiculous stamina.

Because gym defenders have their health doubled, it is unlikely that anything will surpass Blissey. At least nothing that can be placed in a gym.

Good question, but do you really think Blissey is such a beast? I mow them down quickly with strong Machamps / fighters or even Rhydon. I don't find them to be so formidable. Only a pain when defenders toss golden razzberries as if they were peanuts