That is, reportedly, blood from Oswald's bleeding body which he left on the floor of the jail office where he was lying. All of it had to come out that bullet hole in his body. And the blood would have passed through his sweater, and the sweater would have retained much of it. Think about how porous the sweater was and how fast blood clots. If the floor was as bloody as that, then imagine how bloody his sweater had to be. But, his sweater wasn't bloody at all.