I think that the closest translation in English should be:
So in formal English is translated as "I don't know".If you analyze some example of
dunno form merriam-webster they can be translated with
“Whatʼre those things?”
whispered Ron. “Dunno,” said Harry. — J. K. Rowling, Harry Potter and
the Order of the Phoenix, 2003
Cosa sono quelle cose? bisbigliò Ron. "Boh..." disse Harry
From the wiktionary
Puoi pagare il mio biglietto? Boh, devo vedere se ho i soldi.
Can you pay for my ticket? I don't know, I'll need to check if I have the
Any other meaning is purely a conjecture
It's not implicit any other meaning like "I know but I am not gonna tell you", "I don't know and I don't care", "I am confused". So if Italians say to you "Boh" usually they not trying to hide you something or avoid answering, they just don't know.