The use of life detector tests in the CA court system is somewhat controversial. Generally, if the defense and prosecution agree to it and the exam was done by a certified polygrapher, it likey can be admitted. However, a defendent can argue that he has a right to not incriminate himself. Defense counsel could vigorously contest the refusal being mentioned to the jury.
A post is a newspaper blog might not be admissiable under hearsay rules. However, with proper foundation, it could be admitted. Laying the proper foundation is a semester in law school. In general, anyone can post something online using any name whatsoever. Proving that the defendent actually posted the particular item might be another issue entirely. Establishing proper foundation for an entry in a blog might be difficult, but perhaps not impossible with a forensic examination of his computer. Of course, if he used a computer at library or a place like Kinko's it could well be near impossible to trace it back to him. The other issue is whether the prosecution really needs such evidence to convince a jury of his guilt.
As a potential juror I would like to see something that more directly implicates him in the alleged crime. People can refuse to do lots of things. Negatives do not, in my opinion, prove all that much. On the other hand, if ten eyewitnesses testify that they saw him standing next to a murder victim with a smoking gun and that he said he killed the person, and then the authorities bring in forensic evidence that a bullet from that gun killed the person, I might well persuaded about his guilt. Obviously, this example is extreme. The point is that there is such evidence has a more direct bearing on what actually may have happened than taking or not taking a polygraph.
Nevertheless, if the defendent did make such a statement in a blog, he was silly to do so.