There doesn’t appear to be any mention in The Bomber Command War Diaries: An Operational Reference Book, 1939-45 by Martin Middlebrook of a mission/sortie or operation on or around D-Day with the name “Blacktie” and he lists pretty much every raid and it’s date etc..

I’m probably way off the mark but could it just be a reference to the “Invasion Stripes” or “D-Day Stripes” which have the look of a black tie on a white shirt ?

http://fly.historicwings.com/2013/03/invasion-stripes/