Very Skrully, edition 4
If you were an alien race attempting to infiltrate an entire planet, Blackagar Boltagon would be the most delicious choice to impersonate. He never speaks, he lives off the planet but visits frequently with influential individuals, and rules his own kingdom of powered followers.
I will cite my main evidence that Black Bolt is a Skrull from New Avengers: Illuminati #1. Go straight to the scene where the heroes have been captured by the Skrulls. When they escape, Tony Stark, Xavier, and Namor bust out first. The following page features the escape of Dr. Strange and Black Bolt.
Strange does show up awfully quick, leaning on his friends as they escape. But Black Bolt just pops up a couple panels later. Of course, he's not going to say anything. But no one is seen helping him escape, and he shows no signs of fatigue or struggle (even Strange is weak after his release). The King of the Inhumans simply appears behind his fellow heroes and follows them to find Mr. Fantastic.
Each character who escaped from the clutches of the Skrulls that day at least worked up a sweat. They each had a different obstacle to overcome. But Black Bolt showed up as fresh and ready to fight as ever. (Also note the scene where Black Bolt is being held "prisoner" - notice that he's seen with a ray of energy going into his mouth, and that's all. It could just as easily be an image of the Skrulls implanting an energy projection device into the throat of their own Black Bolt impersonator.)
Bendis himself has cited New Avengers: Illuminati #1 as strong evidence of Skrull developments. And don't forget that Hulk whooped Black Bolt pretty damn easy in WWH #1. I know that the Jade Giant is super angry and stronger than ever, but Black Bolt barely put up a fight. How very skrully...
Very Skrully, edition 1
Very Skrully, edition 2
Very Skrully, edition 3