How exactly do obligatory hematophagous organisms that undergo hematophagy use blood to energize themselves? Is it the plasma they are drinking? What kind of metabolism do these creatures have? Is it the Kreb's cycle they are working off of or something else? And can we expect a radically different iron metabolism in them?

Blood for them is just a sort of nutrient source, comparable to human drinking nutritious drinks, and insect blood and the host blood they feed on are not mixed, host's blood gets straight to their digestive system, useful nutrients absorbed, wastes remained. Krebs' cycle is character rooted deep into evolutionary history of life, insects have the same Kreb' cycle mechanism as we have. And for the iron metabolism of course they should be different, but seems to be unrelated to hematophagy, the hemolymph or insect blood does not contain hemoglobin yet with ions.

PDF including digestive physiology of blood-sucking leeches

 

Long story short, they have unique colonies of symbiotic bacteria to help break down the blood, and a digestive system specialized for the consumption of protein.

So, in terms of leeches, they break down the globin protein and use that for energy?

They must have curiously evolved brains if they really aren't working too much off glucose.

I'm guessing the blood cells are not broken down an the amino acids converted into sugars.

Blood has more than just blood cells in it - sugars, salts, lipids, proteins, amino acids, hormones, all in there. By definition, it has everything your body needs, since that's how your cells get fed.