Bob,
Carding is the process in which you lightly and gently remove the excess layer of rust that has built up on the surface of the area being blued/blacked. This is usually done by hand or mechanically with degreased fine steel wool, or extremely fine steel wire brush or what they call a carding wheel...a buffing wheel with extremely fine steel wire. Its not buffing, its lighter than that. The "steel" part sounds harsh, but think "Kiwi shoe brush" fine wire.

Dustin