Headers are name/value pairs, a typical email will have 20 of those. It's possible to copy them if you have received an email from the blog owner or maybe from a mailing list post.
Access to a message or a mailing list post by them won't provide any further advantage.
"We had a specific problem with the way we dealt with SPF records. Dustin didn't set any up, and there was a specific way that Robin Duckett's email server responded that caused us to flag it as a false negative for spoofing."
So, having access to the blog owner's email headers would not have provided any additional advantage to the intruder.