Rule #1 of news coverage is "Never get into a fight with someone who buys ink by the barrel." This will NOT keep these "blocked" outlets from covering the news. They'll not only continue to cover the exact same stories as everyone, they'll a) cover them even more aggressively, and b) they'll cover them using your opponents as sources without you being able to comment, since you've blocked from from you. Furthermore, the White House Correspondents Association just released a statement that they encourage media outlets to share whatever information they get from a White House briefing to outlets that were blocked.
That aside, blocking any news outlets, but especially some of the most respected ones makes you look like you're an idiot, like you're scared and have something to hide, and like you're willing to shred the First Amendment, and if so, then the rest of the Constitution.