Blissey only really helps for HP EV training on top of exp - while you can train other stats on it with power items, the problem is that you'll be getting HP EVs too, which you may not want (e.g. for a fast sweeper), or you may only want a specific number of. I'd advise horde battles for the other 5 stats. They're fairly easy as well - have something with sweet scent and in the right route (e.g. Oddish in the route with the weather institute, Shuppet in Mt Pyre's building, etc) you have a 100% encounter rate.
Also, hordes offer 5 Pokemon at once. Most you get with Blissey bases is 3.
Numbers btw for say EV training in Attack in both cases, assuming power item and pokerus:
Blissey: 3x(1X2) HP, 3x4x2 Attack --> 6 HP, 24 Attack EVs
Shuppet Horde: 5 x (1+4) x 2 = 50 Attack EVs.