Baron Black of Crossharbour or Lord Black, in fact - he's a Lord, not a Knight, and is not entitled to be addressed as Sir (which is actually a lower rank than he holds).
Was not sure if he is still in the House of Lords, there has been several attempts to oust him in the last couple of years, as well as Jeffrey Archer, on the Tory side, or whether the title would stay after being expelled.
In any case, I did “Americanize” the salutation, him being a former Canadian. Correction noted and apology extended.