Here's just a few examples I've found recently :

Union Army Private James (G) Bates enlisted and mustered into Company F 14th Indiana Infantry on 6/7/1861. He was discharged on 10/27/1862. He then enlisted in Battery D, 4th U.S. Light Artillery. In April, 1863 he wrote his father :

I can assure you [Father,] of a certainty, that the rebels have **** soldiers in their army. One of their best sharp shooters, and the boldest of them all here is a ****. He dug himself a rifle pit last night [16 April 1863] just across the river and has been annoying our pickets opposite him very much to-day. You can see him plain enough with the naked eye, occasionally, to make sure that he is a “wooly-head,” and with a spy-glass there is no mistaking him.

His letter was printed in the May 1, 1863 issue of the Winchester, Indiana Journal. The 4th U.S. LA was indeed part of the 7th Corps, Department of Virginia at that time. The Siege of Suffolk, Virginia was April 11-May 4, 1863.
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On March 4-5, 1863, the 85th Indiana fought Confederates at Thompson Station,(or Spring Hill,) Tennessee where they had 8 men killed and 2 wounded. Reporting on the skirmish the Indianapolis Daily Evening Gazette wrote that the 85th reported, During the fight the [artillery] battery in charge of the 85th Indiana [Volunteer Infantry] was attacked by two rebel **** regiments.
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Sarah Venable’s Confederate Pension Application

On July 16, 1929, in Forsyth County, North Carolina, Widow Sarah Venable age 83, applied for a Confederate widow’s pension. Out from her name, in parenthesis, is the notation that Sarah was “colored.” Her husband’s name was John W. Venable. He served in Company H 21st NC Infantry, enlisting, June 5, 1861. There is also notation out from his name that he was “colored.” She was married to John before January 1, 1868. She “has small lot in colored section, worth probably $200.00.” Sarah’s application was disallowed. Another notation says “colored woman, we have no law for this.” On Sarah’s behalf, John Sawyer, also a veteran of Co.H 21st Infantry “made his mark” on a deposition, stating he “was well acquainted with John Venable (colored) who was in Company H, 21st Regiment, that he was honorably discharged from service, and made a good soldier, and his widow is entitled to a pension.”

John’s name doesn’t appear on the Regiment’s muster roll. There was a John T Venable, in Company F, from Stokes County. He died at Fort Royal, Virginia, 9/17/1861. “North Carolina Troops 1861-1865, A Roster,” does list John W. Venable (Colored) Enlisted and mustered on June 5,1861 (no further record.)

John Sawyer enlisted in Co.H on the same day, age 16. He is shown being wounded at First Manassas, July 21, 1861 and later “discharged for disability.”

It appears from the 1860 census, John Venable was a free man. For some unexplained reason, his very large family (mother and siblings) were counted twice in two different Districts of Surry County. In one record they’re listed as “Black.” In another, “Mullotto.” John and Sarah appear in the 1880 census in Yadkin
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Prisoner of War Exchange :

Below, is correspondence, taken from the OR (official records, serial 116, page 0214) between two Union Officers and one Confederate about a prisoner exchange. Note, “four colored men,” “three free and one slave,” “very anxious to go home” are part of the exchange :

FORT WARREN, Boston Harbor, January 25, 1862.

General L. THOMAS, Adjutant-General U. S. Army.

SIR: I have just received the following answer by letter from General Huger to the message I sent him relative to exchanging the North Carolina prisoners of war for the U. S. troops in Texas:

My Government is willing and anxious to exchange prisoners on fair terms, and as the authorities at Washington have permitted it in certain cases I beg your assistance in making it general and thus aid the cause of humanity and civilization.

In another part of his letter he says:

With your assistance, colonel, I hope we can do much to relieve needless suffering to our fellow-countrymen.

Being therefore certain of a reciprocal exchange of the prisoners of war now here I shall immediately require transportation for them by sea to Fort Monroe and to be sent to Norfolk for exchange–4 captains, 2 first lieutenants, 8 second lieutenants, 2 third lieutenants and about 370 rank and file. The four colored men are very desirous of returning to their families in North Carolina. I shall therefore send them unless I receive further instructions in relation to them. Three of them are certainly free and have families South. The fourth man is believed to be a slave yet he is very anxious to go home. He has been to me often begging me to send him home on the first opportunity.

I am, sir, very respectfully, your obedient servant,

J. DIMICK,

Colonel First Artillery, Commanding Post.

HEADQUARTERS ARMY OF THE POTOMAC,
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After-Action Report of Confederate Colonel Thomas Munford on an engagement with the Virginia Unionist Partisan unit “The Loudoun Rangers.” Note a “slave named Edward” joins his master in a cavalry charge :

January 25, 1862.War of the Rebellion: Serial 016 Page 0749 Chapter XXIV. CAMPAIGN IN NORTHERN VIRGINIA.

On the 31st the brigade accompanied General Stuart on a scout to Chantilly, picking up 200 or 300 prisoners. A portion of the Twelfth, under Lieutenant-Colonel [R. H.] Burks, captured one company of the Tenth New York Cavalry without firing a gun.
On the evening of September 1, while our troops were engaged on the Ox road, near Germantown, my regiment was ordered to Leesburg to capture Means and his party.
About 11 a. m. the next day I arrived at Leesburg. Learning that Means was in the town, I cut across from the Dranesville pike and entered the town by the Edwards Ferry road. I succeeded in surprising Means’ party, Means himself escaping. He was supported by Major Cole, of Maryland, with about 200 men, on the Point of Rocks road. Without halting in the town I pressed heavily upon him, and soon succeeded in routing his command after a heavy skirmish, and pursued them as far as Waterford, 7 miles. My command amounted to 163 men, about 40 of which number, including Captain Dickinson and Lieutenants [W. R.] Beale and [A. D.] Warwick, did not join in the charge from some cause not yet explained. Had they followed their comrades in this bold charge I do not think a dozen of the whole Yankee command would have escaped being either killed or captured. As it was, we killed 11, wounded 9 too badly to be sent away, besides some 10 or 11 who escaped badly wounded, and sent off 47 prisoners, including 2 captains and 3 lieutenants.
In this charge Lieutenant J. O. Davis, of Company E, was killed while gallantly leading the advance of his company. Lieutenant John O. Lasley, of Company K, had his arm fractured by a rifle-ball, Sergt. Charles Spears, Company C, was killed. Private N. McGhee, Captain Dearing, of Company F, and John Merryman, of Company I, were badly wounded. It is proper to report that Edward, a servant of Private English, Company K, went into the charge, following his master, gun in hand, and shot the notorious Everhart, who was left in Leesburg, badly wounded.

I am, Major, very respectfully, your most obedient servant,
THOMAS T. MUNFORD,
Colonel Second Virginia Cavalry