Other infusions almost always provide more AR (sometimes more than double), and this makes blessed infusion worthless for most situations excluding parry shields. I mean, 2hp/sec is marginal benefit considering you bricked your weapon. However, there are few exceptions, where blessed gem is obviously the best choice (providing you have high faith):
-Lothric Knight Greatsword
For example, Blessed Lothric Knight Greatsword (24/16/40 str/dex/fth) has 655 AR. At 60 faith it reaches 700. Remember: blessed gem increases >innate< lightning damage the most effectively. DON'T infuse other weapons! S scaling worth nothing if the base damage is halved.