An embarrassing phrase meant to indicate an inquisitive challenge to someone's sincerity as an interrogative (Honest to blog?), or a claim of solemnity as a declarative utterance (Honest to blog!). The phrase found a modicum of popularity amongst idiots back in 2008 thanks to Diablo Cody. It was lame at the time, and has since fallen further out out of usage. It was one of a number of crappy pieces of dialogue that were deemed Oscar worthy by the crusty old members of the Academy, who only awarded the film anything because they wanted to look cool and down with "teen speak" despite not having a fucking clue what any of it meant or the fact that that no one would ever talk like that. The decline in the usage of a term like "honest to blog" is perhaps indicative of a growing cultural awareness of of the crumminess of the film's pseudo teen talk and an acceptance that such bullshit isn't so god damn cute anymore.
For further examples of such out of touch and stupid phrases, see Diablo Cody's shitty post-Juno writing credits (Jennifer's Body, Burlesque et.al.)
"Hey, did you hear Juno won the Oscar for best original screenplay?"
"Honest to Blog?"
"...dude, shut the FUCK up."