Y'all should replay ARR... the way the Blessing of Light comes up in the battles against Ultima Weapon and Lahabrea now is pretty insane compared to what it had been originally.
She actively uses it to channel her Light through us and stop Primal super attacks/break Titan Gaols.
She also uses it to spark LB3 gauge fill against the Ultima cast.
Against Lahabrea she revives us from unconsciousness and essentially gives us Super Saiyan Blue aura complete with 60% HP increase, movement speed increase, and ATP increase. It's a bit more than a "traveler's ward" if you ask me...
Iirc in Japanese, The Echo is referred to as, "Transcendent Power." A tad less mysterious.
Now, I see it pretty much as being, Blessing of Light = Hydaelyn Superpower access.
Echo = Transcendent Soul Potential and ability to hear Hydaelyn. Note that the Echo itself has actually been shown to be able to do things that the BoL has not, such as see the future.
The BoL is only given to Echo individuals, because they have the greatest potential and nearly incorruptible aether already. Pretty sure the BoL is just confused with the Echo by the cast, and they're never not gonna write it that way.
Resonance copies the Echo, and the procedure could copy the BoL too because it's just copying the aetheric waveform of the master subject. You'd likely need even more people to fuel such a thing, though (and somehow trap the active state of it given to you by Hydaelyn).
Anyroad, The Echo has to be what's responsible for resisting Primals, due to the whole first Heavensward arc where the BoL is sealed by Midgardsormr, but also due to Bozja wherein Mikoto's Echo is Resonance copied onto Misjia.