The Blitz referes to the Season 5 episode (aired on Monday Nov 22) of How I Met Your Mother, when one person leaves the group for any period of time and something amazing happens. You can pass on the Blitz from being part of the group when somthing amazing does happen, but another person from the group had to leave between 2-30 minutes prior to that amazing thing happening.
Robin: Barney! where did you go you missed it!
Barney: Missed what?!
Robin: We were in the thanksgiving parade!
(Prior Blitz): yea and i was there!
Barney: Awwww man!!