I finally got an opportunity to see this book. Here's what Riley says on p.305-

"In an extremity so dire, the Confederate government resorted to the arming of the slaves and the placing of them in the ranks of the Confederate forces. This, to say the least, was a doubtful experiment, but it was the only drifting straw in sight to the sinking Confederacy. Accordingly, Gen. John T. Morgan (now senator from Alabama) was commissioned to return to his own state and muster into service able-bodied negro men. Others were deputed to perform a similar service for other states."

There's nothing about troops raised or captured at Selma.

In researching the past, you occasionally find a rare nugget of information, but most of the time you come up with a lot of dead ends.
But you can't find the rare nugget unless you search.