Things tend toward disorder spontaneously. If the two blocks are placed in contact, the hot block becomes cooler while the cold one becomes warmer till they are both the same temperature. Something other than changes in heat content are involved. That other thing is entropy. Criteria for spontaneity of a reaction.
Heat is given off.
In this reaction energy is given off. This is an example of an exothermic reaction. Randomization is also a driving force for spontaneity. If you examine measure the heat change in this reaction you find that it consumes energy. In other words something other than the heat change is driving the reaction.