8
The fieldset does create the block line you describe. If you look at the code here for theme_blockcenter_c.html
le="color: #000000"><?php <div style="padding: 5px;"> <fieldset> <{if $block.title != ""}><legend class="blockTitle"><{$block.title}></legend><{/if}> <div class="blockContent"><{$block.content}></div> </fieldset> </div>
the line is created by 'fieldset' you can simply remove the tag as follows ...
le="color: #000000"><?php <div style="padding: 5px;"> <{if $block.title != ""}><legend class="blockTitle"><{$block.title}></legend><{/if}> <div class="blockContent"><{$block.content}></div> </div>
or as studioC described add an attruibute in the CSS.
If you are not seing these changes, I think you need to clear templates_c and make sure your theme is set to update in admin preferences.