Of course, one big difference between the shooting of Reagan and the shooting of JFK is that Reagan had Secret Service agents who were actually protecting him. JFK had Emory Roberts talking on the phone during the shooting.
The black smudge was added to cover up his hand holding the car phone on which he was talking during the shooting.
But, there is also a black smudge over the lower face of the driver, Secret Service Agent Sam Kinney.
So, what were they trying to cover up there?
But, getting back to the differences between Timothy McCarthy and Lee Harvey Oswald, there was a vast difference in their conditions. McCarthy's wasn't near as bad. I think we can presume he was hemodynamically stable. He didn't have any major blood vessels cut. I'm sure he was bleeding red blood, but it may have been more like he was oozing it; whereas, Oswald was spurting it.
And, because he was spurting red blood, Oswald should have gone down. He should have collapsed, without doing this:
Another important difference is that Oswald lost consciousness whereas McCarthy did not.