The only way to get Seismic Toss on Blissey is through move tutor on leafgreen/firered or emerald. So I guess Ice Beam would be an ok replacement seeing as you cannot get it on Blissey any other way. Actually Alan was sorta right in saying not having Seismic Toss hurts Blissey's tanking ability or atleast the SkarmBliss combo. It is meant to go like this:
Get Skarm into play, spike as they switch to their Skarm counter, switch to Blissey b/c most Skarm counters are countered by Blissey, Seismic Toss their Blissey counter as it switches in (free 100 damage no matter what, whereas they could resist/take neutral damage from Ice Beam making it less effective since Blissey's SPAttk sucks), switch to Skarm and take the physical hit from their Blissey counter, spike again as they switch to their Skarm counter.
Rinse wash repeat till you have 3 Spikes up.
Without Seismic Toss this is alot less effective and gay.
Toxic is no good on Blissey because you'd rather paralyze with T-Wave.
Don't bother BPing to Blissey its a waste of time and it is to difficult. It takes 2 CMs to get her to above average SPAttk, 3 CMs to make her good enough. And a typical scenario for this would be you BP they pull there Blissey counter you don't do enough damage to 1HKO you are forced to switch. If anyone on your team should receive CMs it should be Starmie, with it's move set that at least deals neutral to all types (Ghost, Dark, Poison, Normal and Psychic are the only 5 that take neutral from all the moves on that set I think, therefore its not worth losing that to add Psychic over T-Bolt). One CM and Starmie could sweep a team with some good predictions.
I didn't know Blissey could learn Stealth Rock, can you have Spikes and Stealth Rocks up? If so I suggest Stealth Rock over Ice Beam on Blissey if you can't get Seimic Toss.
Just to through this out threre Herracross is "Teh Sex"
Choice Scarf Heracross, I have nothing left to say gg.