The Blackstaff is literally a black wooden staff. It is similar in most ways to a normal wizard's implement, only where a normal focus would be carved with runes to focus the powers channeled through it, the Blackstaff is devoid of any markings.
Harry Dresden claims to have a distinct feeling that the Blackstaff appears to have a kind of consciousness all its own which is aware of its purpose (primarily, killing), and to want nothing less than to carry it out as swiftly as possible. Dresden also notes that it appears to attempt to exert some kind of control over its wielder, as black tendrils seem to crawl up the arm of the wielder and hurt him, before fading over time.
In the series
In Battle Ground, Ebenezar McCoy uses the Blackstaff in the fight against Ethniu. While in use, it covers his body in a dark shadow. McCoy swept the Blackstaff toward a line of Fomor soldiers and the front rank died in their tracks.
Jim Butcher has said that the Blackstaff protects the user from the backlash a warlock experiences when using black magic, the phenomenon of it "being easier to bend someone the more you yourself are bent." It may also allow a wizard to accomplish a GOAL he believes in without believing the magical method is just.(reference needed)
The staff is implied to be the missing walking stick of Mother Winter. In Battle Ground, using the Blackstaff casts a shadow of "a hideously twisted old woman, complete with the classic witch nose and chin, looking somehow darkly amused."