orthography - Is "blah blah blah" the most common spelling?

What is the most common or correct spelling of "blah blah blah"?

• blah blah blah


• blah blah


• bla bla bla


• bla bla

My question stems from when I first wrote it as "bla bla bla" in an English text, but a friend told me it should have been written as "blah blah" so I decided to ask here.

Before that I had checked it out on some online English dictionary and Google search but I wasn't able to clear it out.