Open up your /content/skins/flat/product_style$.html file, where $ is equivalent the product style you are using (1,2,or 3) .
Adjust the opacity of the square on-hover mouse effect by adding an opacity value in the rel attribute of the <a> tag. The default opacity is 50. Enter a value between 0 (transparent) and 100 (solid).
<a href="{$product.image}" width="{$product.image_width}" height={$product.image_height} class="MagicZoom" rel="opacity: 10"><img src="{$product.preview}" style="border:0px;padding:0px;margin:0px 0px 0px 0px;" alt="{$product.title|htmlspecialchars}" title="{$product.title|htmlspecialchars}"/></a><br>
