Jay, Here is about as good an expaination as possible without resorting to material found in a senior level engineering class. I figure that you have lost interest long ago but mabye somebody else will stumble across this like I did.
Heat exchange occurs in three ways: Conduction, convection and radiation. In your attic the underside of your roof has a thick layer of air between it an the drywall of your ceiling. Air is a lousy conductor of heat. Convection and conduction are just different aspects of the same method, direct contact. So the heat from the sun can only get into your living-room by radiation. Radiant blocks work by putting a surface that has a hardtime radiating heat between the heat source and the heat sink (your livingroom). The heat is not "trapped" anywhere. What happens is the it just finds an easier path. The temp. of your roof goes up a few degrees and the heat that would of gone into your living-room goes back-outside. It works because wood has an easytime radiating and most metals don't. Here is a simple experiment. Put a piece of wood on the hood of a car on a hot sunny day. After a couple of hours grab the wood. It feels warm but not hot. Now put your hand on the car hood. It burns. Why? to get rid of the energy provided by the sun the wood can radiate it away easily. The hood has to get much hotter to do so.
Temperatures are not what has to be in equillbrium, heat is.