Yes, mulch works by blocking light. I used to think it worked by weight, but some more sophisticated gardeners explained to me that mulch and landscaping fabric actually work by blocking sunlight. If there is enough of anything to block sun from the weeds they won't grow. The trick is to find something heavy enough that it won't blow away. Also, I've been told mulch should make a donut around a tree. I the mulch is right up against the trunk it can cause fungus and rotting problems for the tree itself.