How to write ternary operator condition in jQuery?

I would go with such code:

var oBox = $("#blackbox");
var curClass = oBox.attr("class");
var newClass = (curClass == "bg_black") ? "bg_pink" : "bg_black";
oBox.removeClass().addClass(newClass);

To have it working, you first have to change your CSS and remove the background from the #blackbox declaration, add those two classes:

.bg_black { background-color: #000; }
.bg_pink { background-color: pink; }

And assign the class bg_black to the blackbox element initially.

Updated jsFiddle: http://jsfiddle.net/6nar4/17/

In my opinion it’s more readable than the other answers but it’s up to you to choose of course.