I found the following contemporary Southern newspaper report on Olustee.
"We learn from a dispatch to the Savannah Republican that the Federals have abandoned their position on the St. Mary's River and taken to the protection of their fleet. Our loss in the late battle was thirty-five killed and from 700 to 800 wounded. The enemy's loss was between 2500 and 3000."
"The enemy's force is reported to have been 10,000 men of all arms. Our force was about 3500 to 4000. The enemy placed two of their negro regiments in the front and urged them on at the point of the bayonet. They withstood our fire at a distance, but as our troops advanced they retreated. More than one half of the two negro regiments are said to have been left on the field of battle."
Keep in mind that was from a Southern newspaper. I also found some after-battle official reports by participants on the web. I gather a Federal general (Hatch) many months after the battle thought wounded negro troops must have been killed after the battle because there were so few negros among the prisoners in the Confederate General's report. He stated it was known that many wounded negroes were killed on the battlefield, but provides no evidence other than the low number of negro prisoners.
Confederate Gen. Finegan in his report to headquarters said that he had only 3 negro prisoners out of 150 total prisoners (later amended to near 200), but he reported a large number of negroes among the captured wounded (number of negroes not given, but the total captured wounded was 418).
US Col. Langdon's official report states: "I saw many wounded colored soldiers appearing suddenly in front and on my left, without muskets, and it appeared as if they had been lying down and taken the first opportunity to get to the rear. Some of the infantry, while facing the enemy and firing wildly, did not show fear, nor did I see any of them absolutely run off, but groups of them huddled together and did nothing, and many were in this position shot, while they seemed unconscious that they were hit."
From US Capt. Hamilton's official report: "The left wing of the U.S. Colored Infantry could have done little injury to the enemy; they fired very wildly and without purpose. It was not from cowardice as much as ignorance. Their officers appeared to do their duty as brave men, but without self-reliance, and I did not see any of the regiment run, yet they only served the purpose of keeping the enemy in check from charging. They should not be condemned, for I saw nothing wrong that could not be accounted for by want of experience and ignorance of object, apparently."
If the negro troops were the front troops in the Federal advance and later simply huddled on the battleground confused, it is not surprising that a high percentage of them were wounded. Maybe they didn't know what to do. Col. Langdon did not say the negro troops had surrendered or that firing from Confederates was unjust.
Perhaps you could point me to some other references. I don't doubt that some negroes may have been killed after the battle, but I didn't find support for it.