Most in-game battles can be won simply by using brute force by over leveled or super effective damage. Blissey's best role is as a Special Sponge and Cleric, and is usually extremely tardy in damage output, with the sole exception of Toxic (which is casually Full Restore'd in later battles anyway) and Seismic Toss (which is most powerful at level 100 anyway). And you yourself have access to Revives, Hyper Potions, Full Restores etc.
Further, the vast majority of battles are single battles, so you can't even use Blissey to immediately aid your offensive Pokemon, and you're better off using items over Wish, etc.
For competitive sets in-game (at the various Battle Facilities), its prevolution Chansey does a much better job with Eviolite, and is generally helpful in beating more powerful AI by means of Minimize stall.