@cel is right: ipython notebook clear cell output in code
Using the clear_output() gives makes your Notebook have the jitters, though. I recommend using the display() function as well, like this (Python 2.7):
from random import uniform
import time
from IPython.display import display, clear_output
def black_box():
i = 1
while True:
clear_output(wait=True)
display('Iteration '+str(i)+' Score: '+str(uniform(0, 1)))
time.sleep(1)
i += 1