It wouldn't have worked and even then, there would have been a political price. First, Washington leaked like a sieve back then; details would eventually leak out. Second, the TV news was coming into its own as a dominant news source. Imagine Cronkite signing off with "And that's the way it is, Tuesday January XX, 1967, the 6XXth day since the media blackout on a war in Vietnam that has so far killed Y Americans." They could black out details of the war. They couldn't black out the death toll. Finally, I'd throw in that the death toll plus a black out would make imaginations run wild. That means more resistance to the draft from draftees and, probably more importantly, parents of draftees.