Edited: Bad answer removed.
Here is the working code:
function custom_blogname_rss($val, $show) {
if( 'name' == $show )
$out="Custom Blog Name";
else
$out = $val;
return $out;
}
add_filter('bloginfo_rss','custom_blogname_rss', 10, 2);
Don’t forget to change Custom Blog Name to something useful.
Put that code into a plugin.