In the episode "A Private Little War", in the opening sequence, Spok is shot in the back by a native with a flintlock musket. His Vulcan green blood is seen both on the front and back of his shirt. Yet no bullet holes are in the shirt! I guess the Klingons gave the native phasing bullets that would bypass clothing, eh?
Now that's really science fiction writing for you. A phased bullet??? If some civilization in the Star Trek universe has the technology to change the phase of matter, then they could eliminate the use of simple lead bullets and use pure energy as a weapon.
While this slip-up is true, it was also 60's television. If I remember correctly, blood was rarely shown, even on the multitude of Westerns and war series airing at this time. I would think that the only reason they showed blood in this instance is because Spock's blood was green. If they couldn't show much blood, I can't see them bothering with a bullet hole. But it is one of those things you catch and end up scratching your head as to how they could do something so lame.
This is more of an indication of just how more strict t.v. censorship was at the time than it is now.