There are two ways to do this. First is like this:
while True: # Loop continuously
inp = raw_input() # Get the input
if inp == "": # If it is a blank line...
break # ...break the loop
The second is like this:
inp = raw_input() # Get the input
while inp != "": # Loop until it is a blank line
inp = raw_input() # Get the input again
Note that if you are on Python 3.x, you will need to replace raw_input with input.