Just to be clear, we may have played it wrong from day 1 . This was an LFR PC, so multiple DMs missed it.I didn't see where pemerton posted them, but searching on ye olde google sure isn't much help. I can easily find that blast affects x-squared number of squares in front of the source provided line of effect exists, where x is the blast's size (e.g. a blast-3 affects a 3x3 area, or 9 squares), and doesn't affect its source. But that's really about all I can find.
Can't quickly find any references to ammunition requirements; as the examples of how blast works all reference AoE spells or effects e.g. dragon breath or burning hands.
Re Blinding Barrage in particular, I did find that the per-hit-target damage was errataed down in 2011 to just be your Dex modifier instead of 2W, if that matters at all.
Reading that Blinding Barrage write-up literally, if I was strictly playing by RAW-to-the-letter I would have interpreted it the same way as Oofta: that you only need to throw one light weapon to get the effect because it specifically states "a light thrown weapon", not pluralized. More likely I'd have assumed this to be a typo, and houseruled it to state you need to be wielding (or at least have very quick access to) at least one for each target.
The only actual reference I found that directly relates to this question is, of all places, in an online google preview of Dungeons and Dragons 4th Edition For Dummies (Slavicsek and Baker) where they in fact pluralize "daggers" in their short blurb about the power. (they rank it there as #5 in the top ten Rogue powers)
On the other hand magical daggers returned automatically so that may have overridden the general rule.
Which doesn't really change anything when it comes to the power. It's still pulling and throwing multiple daggers per second (assuming you did anything else during the round) or had to pull a Hawk the Slayer with a hand crossbow. Well, that and the only real reason it could be done once per day was game balance.