The 'Blitz' or 'Blitzkrieg' in full, is a German military strategic term, meaning 'Lightning war', and it was a popular Nazi war strategy in 1939-1940s (i.e. World War 2). Although this was used by the Nazi German forces, via a massive air deployment and attacks in a number of campaigns in this war, today it generally is used to refer specifically to the German air 'Blitz' on Britain between 1940-41, which ended with the famous Battle of Britain and the British RAF's victory in the air, over its home territory.
In World War 1, air attacks (the airforce technology still rather primitive then) were made regularly on Britain by Germany but these were NOT of the scale and intensity of World War 2, and thus, proerly speaking, the term 'Blitz' is not used with reference to World War 1.
Hence, when we speak of 'Blitz' in war/military contexts and regarding especially air warfare, today, we are invariably talking about the World War 2 'Blitz'.