For instance, I would always use a single syllable as a past tense: Then he blest him. On the other hand, usage has long hallowed the expressions every blesséd one of them and when is the blesséd event? No doubt some folks would say blest in every case.
But then, what about these words? What about cursed and its down market cousin, cussed, not to mention the derivative accursed. How would you pronounce the following: I cursed him. Grab the cussed/cursed thing and let's go! He is known as The Accursed. How about deuced in That's a deuced hard thing to do. In the old movie line, how do you hear the Indian say, White man speak with forked tongue?
For that matter, we say aged cheese but an agéd man -- or do we? Do we not occasionally slip an extra syllable into supposed -- especially when we turn it into supposedly?
And then there are the -ed words built out of archaic verbs we no longer use, words like jagged, ragged, and rugged. And, of course, naked. I could say that some is nak'd, but nobody does; no, it's always nakéd, from Old English nacod, past participle of nacon, "to strip bare."
I'm sure I could dig out more examples when those who are sticklers for blest pronounce the -ed as an extra syllable, but these should suffice. In the meantime, y'all have a blesséd day, ya hear?