Check if current node meets conditions, then alter a block's contents?
You can check the content type of current node via preprocessing functions in your theme, but there might be another (simpler) way to do what you want: Drupal adds various information about the current page in the body classes, so you have more options for theming.

To select a specific menu link on all nodes of blog content type you can use the following CSS class: body.node-type-blog and a unique menu link ID (#menu-1000-1).

Add that line to the place where you define the active state of menu items. In your case this will be (in your superfish CSS file):

body.node-type-blog #menu-1000-1 a,
a.sf-depth-1.active, .active-trail a.sf-depth-1 {




Another quick solution would be using jQuery:

if ($("body").hasClass("node-type-blog")) {
  $("#menu-1000-1").addClass("active-trail");
}




Check also Context module for more complex use cases where you need to define a section on your website and apply certain theming/functional changes.