I think that Dazzling Gleam isn't totally unviable on Blissey. It has pretty good coverage in UU, hitting important targets like Hydreigon, Honchkrow, Mega Absol etc for major damage despite coming from Blissey's average base 75 special attack. It also nails fighting types, Blissey's main nemesis, on the switch in, discouraging them from switching in repeatingly. However, due to Blissey uninvested special attack stat, it actually does less damage to extremely bulky fighting types like Assualt Vest Machamp. It also loses the ability to effectively run Toxic due to total inability to damage Poison and Steel types. While Dazzling Gleam on Blissey may be a generally inferior option, it has some occasional uses and isn't THAT silly.