I've found "Blessed are you, Simon son of Jonah, for this was not revealed to you by man." in the Bible.

"Blessed are the poor in spirit, ~~"

"Blessed are those who ~~"

These are the same structure, I think.

I've found explanation about the inversion.

Original structure of "Blessed are you ~" is you are blessed.

I wonder if this is common inversion or just in some old English or?

'Blessed is' sounds better to me. I'd be guided by the noun that is closest.

The inverted forms "Blessed are...", "Blessed is..." etc. are old-fashioned or literary forms that are nowadays encountered only in religious or poetic contexts. Here I would normally pronounce "blessed" with two syllables: "bless-ed".

"is/are blessed", on the other hand, may be heard in modern English; for example "We were blessed by the priest", or, less religiously, "I'm blessed with good health", "We were blessed with good weather". Here "blessed" is pronounced as one syllable, like "blest".
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I have a related question. I was asked to sign a guestbook recently, and the message I left was as follows: "Blessed are this house and those who stay here". I was criticised by my fellows, they said I should have written "Blessed is this house and those who stay here". Were they right? My logic is that the inversion, if written in the more conventional way, would have read: "This house and those who stay here are blessed.", and hence the plural conjugation of the verb must be maintained in the inversion. What do you think?
 Clive's reply was promoted to an answer.