You can use Id instead of class for this.
As Id are supposed to be uniqe in DOM, so any css applied to that control id present in master page, will only be added to that id and not to the content pages.
#myMasterPageControl { color: red; }
.myChildPageControl { color: green; }
Div in master page:
<div id="myMasterPageControl">Some Text</div>
Div in content pages:
<div class="myChildPageControl">Some Text</div>
<div class="myChildPageControl">Some Text</div>
<div class="myChildPageControl">Some Text</div>