Edit in JSFiddle

$(".button").on("click", function(event){
	$("#box1 > span").toggleClass("blue_bk");
});
<div class="button">Click</div>
<div id="box1" class="box">
   <span class="text">This is the text block</span>
</div>
<div id="box2" class="box"></div>
<div id="box3" class="box blue_bk"></div>
.box{
  height: 100px;
  width: 100px;
  background-color: green;
  float:left;
}
.blue_bk{
    background-color: blue;
}
.button{
  width:100px;
  background-color: lightgrey;
  padding: 5px;
  border-right : 1px solid #000000;
  border-bottom : 1px solid #000000;  
  text-align:center;
  cursor: pointer;
}
.text{
  color:white;
}