Hi,
I used a black kingston class 4 card for taking short clips (less than 2min) on my 60D and I notice that the card access light blink every second (approx.). This seem to indicate that the camera buffer can store around 1s before it's need to write to the card and able to transfer the data before the camera buffer full again.
By the way, FI, I do a simple test (copy a 3GB file to the card and record the time took for 10 times to get the average write speed) on my black kingston class 4 4GB/8GB card and they all can write at a speed of more than 8MB/s, but I notice one of my previous blue kingston class 6 is actually slightly slower than the black kingston class 4 card.
Have a nice day.