I don't believe I have ever made a claim about numbers and, quite honestly, do not believe that such a claim could ever be made beyond rough estimations. The reason for this is due to the extremely incomplete records on the confederate side in general.
Is there any mention in U.S. Army accounts of that regiment of black rebels near Saylor Creek?
In all honesty, I do not know. There are written accounts of it quoted at the site in addition to that newspaper account, but I did not write down their location in the records when I was there. There are about 100 volumes in the War of the Rebellion set, and the sum of them represents only about 10% of the entire US government records collection on the war. They have CD-roms of it that are searchable by word, but these are expensive and I do not have one. The remainder of the records are mostly in the Union Provost Marshall collection, which is almost entirely on microfilm without any particular index except for, in some cases, last name. So if I had to take a guess, I would say yes - it is likely that an account of that battle exists in the official records and it may be the account that is on the historical marker there.
What was its designation -- e.g., the 1st Virginia CSA Colored Regiment? Which Confederate officer commanded it? Which other Confederate officers were in it?
They were called "Turner's Richmond brigade" after their location of muster and commander, Major Thomas P. Turner. The group was mustered in Richmond on March 11 and March 21st at a site recorded as "Smith's Factory." The surviving records of their muster don't list a numerical designation and it is possible that they did not have one (Another group mustered at the time was known similarly as Chambliss' Winder-Jackson Battallion, so this was not uncommon).
As best I can tell, they were under the command of Fitzhugh Lee at the time of the battle. The combat occurred on April 4 between Amelia and Sailor's Creek. It consisted of two assaults by a union calvalry unit (The first was repulsed and the second pushed through).
Did the black veterans of the regiment get Virginia state pensions as did white rebel veterans?
I haven't checked the pension records for Virginia, but it is probable. Several hundred blacks in other states recieved confederate pensions and I do have lists of those records. They are especially frequent in Tennessee, which had accepted blacks into the state militia since June 1861.
Where is the roster of the regiment?
Either buried deeply amidst literally millions of documents in the national archives, or lost. The only records they have found to date, aside from accounts of the battle, repeat the muster details I stated above.