“Blue Bloods,” like many police procedural shows, may be too formulaic to get a lot of attention from Emmy voters. These voters often want characters who struggle with extreme internal obstacles, and when you have a different case every week, it's hard to get many long-term plotlines throughout the season.
After a decade, Emmy voters have to know about “Blue Bloods,” and if they weren't going to nominate it in its earlier seasons, then they probably aren't going to now. Emmy voters want something fresh and innovative, and “Blue Bloods” just hasn't done anything outside-of-the-box enough to grab their attention.
Friday nights are a tough time for any show to grab new viewers, and “Blue Bloods” has pretty much stayed in its Friday timeslot since it first aired. One reason “Blue Bloods” has been passed over may be because the show hasn't had a chance to bring in the fresh audience that gets awards buzz going.