Slimmer Pickings: This document drawn up by the auditor for Virginia identifies the number of black men in the state, both slave and free, from the ages of eighteen to forty-five, who would be fit for service in the military as of March 25, 1865. Twelve days earlier, the Confederate Congress had passed a law permitting African Americans to serve as soldiers.
According to the 1860 U.S. Census returns, the counties and cities on the list contained 65,720 male slaves in age range of eighteen to forty-five years old. But by March of 1865, the number had dwindled to 25,697 men.
Source: Encyclopedia Virginia
The Encyclopedia Virginia has some interesting information on the Black exodus in Virginia, and its impact on the policy of enlisting enslaved people.
In late March, 1865, the CSA government legislated that enslaved men could enlist in the Confederate army. The following talks about the loss of slaves to owners during the war, which would impact the pool of black men who were available for enlistment:
Yet those slaves living near Union armies did routinely take advantage of the opportunity to run to freedom, and the likelihood of families escaping together increased over the course of the war. Slaveholders near Union armies in northern Virginia, southeastern Virginia, and along the Chesapeake Bay reported frequent mass exoduses. The Rockingham County Court suggested that their proximity to Union soldiers in West Virginia made slavery "a volunteer matter altogether."
When slaves could leave their owners' homes and be safely within Union lines after only a few hours' journey, they were far more likely to run away. Close proximity to Union armies provided more opportunities for women, children, and elderly slaves to run away, although state and Confederate government officials were far more troubled by the departures of able-bodied young men.
Documents circulating between the governor's office and the Confederate army near the end of the war illustrated how successful Virginia's slaves were in achieving freedom through escape. Late in March 1865, Governor William "Extra Billy" Smith sent General Robert E. Lee a list enumerating all black men, slave or free, within the state of Virginia. This list relied on returns from both the county courts and the state tax assessors, and Smith assured Lee that the list, while probably overestimating the number of remaining slaves in each county, was relatively accurate.
Slave owners in the fifty-nine counties and three cities on Smith's list held only 25,697 male slaves between the ages of eighteen and forty-five in March 1865, an estimate the governor himself admitted was probably too high. According to the 1860 U.S. Census returns, those same fifty-nine counties and three cities had contained 65,720 male slaves in a comparable age range.
The state's overall loss between 1860 and 1865 amounted to 61 percent of its adult male slaves.
Note that, these black men had not necessarily left the state. Rather, most were behind Union lines and therefore inaccessible to Confederates. Some number of black men might have been removed from the state to prevent them from escaping.