The Blitz failed:
The reason the Blitz "failed" (I like to think it stopped), was Hitler decided to break his bond with Stalin and invade Russia. Something I'm sure he felt necessary, even though it was colossally stupid. Having a nasty like Stalin standing so close behind would have worried the hell out of me! The invasion of the USSR required the bulk of the Luftwaffe to move East and brought the Blitz to a complete end. Mind you the advent of radar controlled AA guns and detection stations also gave the British a new way to defend themselves. BTW, my uncle flew Hurricanes during the Battle of Britain, before transferring to the fledgling CRAF where he flew Spitfires providing bomber escort in Sicily and Italy, where my father was fighting. His view was the RAF was pretty darn lucky. The German pilots had to fly over the Channel before they could start fighting, and therefore started off with much less fuel, and were already beginning to tire from flying. The RAF was young, determined and brave, but most of all they were fresh!
Now I'm in perhaps a unique situation. I have a Blitz survivor living with me. My mother in law was bombed out of one home, and evacuated to Canada for the rest of the war. Her mother was bombed out of two more homes (those 'dumb V1 & V2 rockets), and survived until 1999. I've not just talked to survivors, I married into the family! Her recollection is that had Germany kept up the bombing, it would have broken the British. The simple fact is the respite from bombing gave the plucky Brits renewed resolve to keep the battle going.
I've often wondered what would have happened had the US maintained Army and Navy bases in Europe after WWI. They might have had a greater sense of impending doom, and perhaps wouldn't have been quite so unprepared in the Pacific and therefore stopped the Japanese from attacking in the first place.. It may have altered Hitler's plans to invade West, and he may have tried for Russia sooner.