How much and what kind of radiation passes through a cloud?
Here is not a detailed article, but one you might find some informative thoughts in:  http://www.americanscientist.org/issues/pub/sunshine-on-a-cloudy-day

On a basic note, any generalization starting with "clouds block x or x%" are far too general to compare actual percentages.  It assumes a cloud is a cloud is a cloud.   They are not.  What a cloud actually does to block or not block electro-magnetic radiation is not an absolute.  It varies by type of cloud, thickness, altitude, angle of the sun and likely many other factors.  As a generality, clouds block a part of visible, UV and IR light depending on a number of factors especially depth of the cloud.  Some of the EM waves are absorbed, some reflected and others pass through.  The UV end of the spectrum tends towards passing through while the IR end is more prone to being reflected which helps to create a greenhouse effect.  These are often presented as absolutes, but are actually generalities.