You can add a ColorMatrix-Node and use the following values:
( taken from http://www.martinreddy.net/gfx/faqs/colorconv.faq )
0.299 0.587 0.114
-0.147 -0.289 0.436
0.615 -0.515 -0.100
Now, this will look a bit different from Shake, since Cb and Cr will be in the range of -0.5 and 0.5. Therefore, add an Add-node and add 0.5 to Green and Blue Channel. But, if you just want to blur CbCr, add a Blur-node and Green and blue with whatever value you like (like 1 in width and 0 in height), and use the inverse ColorMatrix to get back again. (Copy the ColorMatrix-node and click on "Invert")
Is this what you are looking for?
/H