I have no explanation for this behavior, other than that they just seem to like bluebirds. I had a captive house finch in the studio for nine years. He had a mirror he loved to look into. Then I cut out and plasticized a photo of a bluebird, and that finch slept next to it, pecked it gently until it wore to white.
You are annoying. Please leave.
(finch takes the not-so-subtle hint)