Make a colour blink

[cruzud at gmail.com]

Hello. I'm trying to make a colour blink when an event occurs.
But the program is displaying only the last colour:

self.bttn.bind('<ButtonRelease-Return>',blink)

def blink(self, event=None):
    lbl['background'] = 'red'
    sleep(0.5)
    lbl.['background'] = 'SystemButtonFace'

Can you help?