To race the black boo in the "You Vs. Boo" mode, race the normal boo, get to the end of the race before the boo and hit the flag dead on. This will not work if you hit the pole either above or below the flag. The option of racing the black boo will only be activated in that level and can be deactivated the same way you activated it, by hitting the flag dead on before the boo reaches it.