A misnomer. Although the German air raids picked up the label very quickly, the London "Blitz" was in fact the precise antithesis of what the Germans understood by Blitzkrieg warfare: "lightning war", a war of speed and manoeuvre in which air power was vital as a backup to ground troops, but still only secondary. The sogenannte Blitz was, in fact, an admission of the limitations of the Blitzkrieg when confronted by a 20-mile wide anti-tank ditch. The assault on largely civilian (or, more precisely, economic) targets - London, its institutions and its docks - was a return to the principles of static siege warfare as practised since ancient times, an attempt to break the morale of a defender by sustained bombardment in order to avoid any need for a final ground assault, without any of the elements of speed or surprise. The main factor that linked the bombing of London with the Blitzkrieg on the continent which preceded it was that air attacks on civilian targets (particularly the transport network) and civilian involvement in general formed a fairly conscious element of Blitzkrieg strategy as a way of hamstringing the defender, trying to make rapid troop movements on roads crowded with refugees.
Although the word is, in history and local mythology, primarily associated with London, it was also applied passim to the Luftwaffe's night bombing of many other British cities and industrial centres throughout the war, although this was on a much reduced scale after the German invasion of Russia in 1941.