On your info file you are defining two block regions:

regions[leftside] = Leftside
regions[rightside] = Rightside

but I think this theme uses only one region for blocks, the rightside (the other region where the content is printed is not a region for blocks).

It is not wrong to have a leftside region intended to go above or under the content (though a better name on that case would be content for the region) but in my opinion for something like that CSS would be needed to make blocks on that region to have same width as content and not the rightside blocks width. That's why I think it is not your intention to have a leftside blocks region.

So I think the leftside block region is not needed. Besides a side effect of having that region, when you first enable this theme all enabled blocks go into that region and it is not easy at first, looking to the sample image of your theme, to understand what is wrong. At first glance someone thinks there must be any CSS error so blocks are rendered above the content. Most themes are going to name those regions left and right (its computer readable name) so when the theme is activated coming from another theme even blocks on right sidebars are going into your first defined region (leftside) because you don't have a right region but a rightside one.

If that leftside region is deleted from info file, of course it has to be deleted too from where it is printed, because it is not needed:

<?php print $leftside ?>

in page.tpl.php if I'm not wrong.