Yes, half Guinness and half Champagne is a 'Black Velvet'. (although I've seen folks use cider)
A 'Black and Tan' is Guinness and Bass ale. - Use Harp Lager instead of Bass, and it's a 'Half and Half'.
Lately, Guinness have been promoting an 'All-Irish Black & Tan' made with Smithwick's Ale.