Style block based on configuration setting
I'd probably just use the Block Class module and assign a classes like block-left and block-right.


  Block Class allows users to add classes to any block through the
  block's configuration interface. By adding a very short snippet of PHP
  to a theme's block.tpl.php file, classes can be added to the parent
  &lt;div class="block ..."&gt; element of a block. Hooray for more powerful
  block theming!