In your examples, in the first, the word "blessed" is an adjective. In the second, it's the past participle of "to bless." I don't know of any rule. I think the adjective is usually pronounced as two syllables. I think the past participle is usually pronounced as one syllable. The dictionaries I checked are unclear and ambiguous. They seem to be saying that the adjective can be pronounced either way--and so can the past participle.

'Blessed are the poor in spirit.' Here, you could either say it as one or two syllables, like Dan said. The two-syllable way, I associate more with Bible passages...two syllables, to my ear, sounds more old-fashioned or maybe profound, but it fits a Biblical context. 'God blessed them.' Technically, you maybe could say 'blessed' as two syllables, but it would sound a little funny or old-timey. Like a peasant from the 1700's. For general use, pronouncing it as one syllable would be a good strategy...like in the following examples: The priest blessed the child. Having been blessed, he felt safe for the journey ahead. I have a great family. I'm truly blessed.

The pronunciation difference you've noticed in the word "blessed" in the two sentences is related to its usage as either an adjective or a verb. In the sentence "Blessed are the poor in spirit," "blessed" is used as an adjective to describe the state of being blessed or favored. In this case, it is pronounced with two syllables, like "bles-id" (/ˈblɛs.ɪd/). This pronunciation follows the typical pattern for adjectives ending in "-ed." In the sentence "God blessed them," "blessed" is used as a verb in the past tense, describing an action that occurred in the past. When "blessed" is used as a verb, it is pronounced with one syllable, like "blest" (/blɛst/). This pronunciation is a contraction of the word "blessed" and is commonly used when the word functions as a verb. In general, the pronunciation of "blessed" depends on its role in the sentence. As an adjective, it has two syllables (/ˈblɛs.ɪd/), and as a verb, it has one syllable (/blɛst/).