Credits are a very complicated business, and the credits we see on screen have a lot of significance off screen as well.
Crediting someone as a "star" in a show instead of "guest star" (or "special guest star," as Greg Jbara is credited in "Blue Bloods") means changing their union-mandated pay scale -- it's a promotion in more than name.
That gets complicated in a show like "Blue Bloods," which has a large ensemble cast. Donnie Wahlberg, Bridget Moynahan, Will Estes, Len Cariou and Tom Selleck are all stars of the show. That's a lot of people at the top pay tier.
And then there's the rest of "Blue Bloods'" sprawling cast list: Amy Carlson, Tony and Andrew Terraciano, Sami Gayle, Abigail Hawk and Jbara have all appeared in almost every episode of the show as well, but they don't get their names in the opening sequence at the start.
But of course, that's not all that separates the two groups; there's also actual screen time to consider. Though Jbara has appeared in nearly every episode of the show, he hasn't been in as many minutes of it as, say, Wahlberg or Moynahan.
There are certainly precedents of second-tier (generally called "recurring" because it's a little less insulting) cast members being promoted to star, even if it's only temporary. "The Walking Dead" does this a lot, for example, probably because stars keep dying in that show.
This was likely the case with Jbara. I say "likely" because, after digging around quite a bit, I couldn't find an instance of Jbara being credited above his usual "special guest star." I'm taking your word for it, because I couldn't go through all 149 episodes of the show.
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