One thing I haven't noticed anyone mention yet is that the Resonant (or at least Fordola) got their powers by copying Krile, who possesses both the Echo and the Blessing. That kinda muddies the water further when trying to figure out the difference. Zenos also doesn't help solve the debate, as he's shown to have other unusual powers from being the great-grandson of Emet-Selch, an unsundered Ancient.

The impression I get is that the Echo is the mark of a reincarnated Ancient, which provides protection from tempering (among other plot-related powers, like our flashback viewings), while the Blessing allows us to call on Hydaelyn's power more directly. But the writing has never been terribly clear on the difference, and we've met so few people with a genuine Echo (Minfilia, Arenvald, Ysayle, Krile, Mikoto, Ryne, and the Warriors of Darkness are all I can remember, other players aside) that we don't have a huge sample pool to pull from. Kinda feels like even the writers forgot the difference after a while.