Learn English – the part-of-speech of the word “O” in the exclamatory sentence

To summarize the discussion that has happened in the above comments:

1. There is no general single-word replacement for "part of speech" that has any real usefulness.
2. If you want the average English speaker to understand what you mean, just say "part of speech".
3. If you are speaking with academic grammarians, it may be more appropriate to use the term "word class", and in some very specific circumstances, if everyone involved is paying attention, you may be able to get away with just "class", which is a one word replacement, though it is really just an abbreviated form of the longer, two-word term.

Copied directly from TRomano's comment that should have been an answer:

It is the past participle of the verb 'bless' used adjectivally in a sentence that employs inversion. The man is blessed... It is a predicate adjective

Inversion just means the sentence order is switched around from what you would normally expect, so the subject is at the end and the predicate is first.

I'll add that in "(blessed) is (the man who walks not in the counsel of the wicked)" 'is' is a copula, which makes 'blessed' a predicative expression.