Technically speaking, "blood is plasma, plasma blood" (as someone wrote) is _not_  a complete identity. Plasma is a hyponym of blood. Blood is a hypernym of plasma.
Since "Beauty is Truth, Truth Beauty" was not intended as a tautology (no matter who the speaker is), the problem is just to understand which is the hyponym and which is the hypernym -- that is, is Beauty a part of Truth, or vice versa? It might even mean that, in turn, one leads to the other, being first hyponym and then hypernym to the other.
I don't think Keats meant to state their total identity through that line. Yes, they are very abstract concepts -- that is why the identity has that wonderful cosmic power.