As I understand it:
Lightning damage = overload of electrical energy
Radiant damage = overload of spiritual energy
Psychic damage = overload of mental energy
Force damage = overload of magical energy
That is the common understanding, but it doesn't quite fit EB. The follow through would be that "life is magical" (and thus closer to being overloaded by force damage) and objects (including magical ones) aren't magical (thus are farther away from being overloaded by force damage). Magical items seem like the complication to that theory (and it seems like detect magic would have less utility if "life is magical").
Don't get me wrong, I am fine with Schrödinger's blast that is harmless light/force blast until it hits something (and then becomes one or the other based on what was hit). It just seems like using life as a weapon is a very GOO thing to do, and the perverse nature of fey and fiends might find it interesting as well.
My interpretation is less that life is magical and more that living things can manipulate magic (though not all of them have the skill or desire). Since creatures have a more complex relationship with magic than ordinary objects, they can be harmed by magic in more complex ways. In this case,
eldritch blast is pumping them full of something weird and sinister that makes them start channeling magic force uncontrollably for a couple seconds. The blast itself isn't the magic force; it's the cursed, eldritch energy that makes the target harm themself with magic force. Magic items aren't vulnerable to the blast because they're just tools built to manipulate magic in a couple very specific ways, and thus not sophisticated enough to be hijacked (though I might make an exception for sentient ones).
That's how I'd fluff it at any rate.