Confide > Psych Up
First of all, Confide has more PP than Psych Up. Second, Blissey doesn't require any Special Attack move except for Shadow Ball but that's incredibly niche and useless most of the time.
Blissey has to check a lot of stat boosting Special Attackers such as Geomancy Xerneas, Calm Mind Primal Kyogre, Arceus formes, etc. etc. Psych Up having incredibly less PP of 16 means that by forcing switch-ins and having the PP exhausted, the attackers can easily set-up to infinite and break Blissey without much trouble.
Confide doesn't suffer the same thing, however. It has same PP as Calm Mind and twice of Geomancy, which means the special stat boosting swepers can't stack up SpA boosts enough to break Blissey.
On the other hand, copying SpA boost doesn't really help it. Blissey's SpA is awful no matter how many boosts, the chances of Blissey counter-sweeping is very rare. If you want to copy stat boosts, then use Ditto, not Psych Up Blissey.
Lastly, Blissey requires a lot of moves: Soft-Boiled, Aromatherapy / Heal Bell, Seismic Toss, Confide, Toxic, Teleport, etc. With all these options limited to only 4 choices, it's not possible for Blissey to use an attacking move that isn't Seismic Toss so the SpA boosts won't help in any way.
Hope this helps!