To assess this weapon, you need to see it for what it is.
1) This is a weapon, it gives a fight boost and some random bonus damage. Random bonus damage is not hugely viable.
2) This is a Bless token generator, as of yet one of rather few, it will generate an inconsistent volume of tokens (more consistent at higher playercounts, but then it's also harder to maintain a high density of the tokens in the bag).
So, Blessed Blade is a card that does 2 things, cards that do several things are allways interesting because if just 1 or more of the mechanics are useful, the card will see play.
Bless tokens vary a lot for different playercounts. A lone Bless character in a 4 man team isnt able to keep up the bless volume, cashing in on a Radiant Smite for example is tricky, for the big team bless tokens will generally be quickly found and generate a few extra successes per map, which is nice. On the flipside a Solo bless character (or someone in a duo) can hog all the tokens for synergies (Radiant Smite).
As a weapon, the Blessed Blade depends on a high percentile of Bless tokens to be lethal, otherwise it's just a fancy knife! The blessed blade synergizes with it's own tokens, so as explained above, a solo character is more likely to be able to build a bag full of Bless and stand to score bonus damage on half or more of all his tests.
TL:DR from the above. Blessed Blade is essentially a team-support weapon in big groups, and a bit more of a killer in small groups. But when is blessed blade a good choice?
Blessed Blade is a good choice when you need a sidearm or backup weapon, often you will find an enemy with an awkward health total or a very low health total, where taking a series of random swipes with the Blade is more useful than swiping with a Timeworn Brand (because it generates a bless token) or conserves your charges on Enchanted Blade, Shrivelling or any guns you might have. You might take 2 in deckbuilding, or stuff just 1 copy in your deck because it's just a backup weapon.
To synergize the blade, any bless generation is good, Sister Mary is a natural fit, I think at least 1 copy is almost mandatory for her. Any cards that build up the token bag with bless (Book of Psalms for example) will really buff up the blade and cause you to land 2 damage hits.
A trick to use when you use this weapon is to find circumstances where you'dd be fine with or without bonus damage. Often this involves playing 2 weapons together, for example Enchanted Blade and Blessed Blade together. If you encountered a Ravenous Ghoul you could lead an attack with the Blessed Blade to see if you got bonus damage and exhaust for a bless token, then finish with Enchanted Blade one way or the other. Then with the same sword combo you might encounter a bigger Icy Ghoul and this time swipe twice with the Blessed, and if the Ghoul survived, finish with Enchanted again.
The bless tokens in of themselves are useful in general, especially if you have some mediocre stats at 2 or 3. With a decently bless filled bag a 3 character stands to pass a couple tests that you've given up on in the mythos phase, 3 will go much further when trying for clues and 2-3 stands to get out of trouble in a coupele tries. Building towards this game-state is a useful benefit of Blessed Blade by itself.
Last but not least, as of writing this we've only seen a few Bless cards, and Blessed Blade + Sister Mary + Radiant Smite is already a rather cool lineup, I cannot wait to see what else is coming. Every new method to generate Bless tokens is going to make Blessed Blade that much more lethal and every new source of Bless tokens will make it that much easier to set up the next bless-themed combo card.
Blessed Blade will see play, and you should try it, even if only to push the Bless gimmick.