PDF Transparency Model defines 16 blending modes. These modes were inherited from Adobe Photoshop. Only Normal blend mode is intuitive. Other modes are not intuitive at all. They exist to provide some special effects. The idea is that the user tries all the modes and selects the one that produces the desired result.
To get the feel of PDF Transparency blend modes one needs to play with them. Adobe Illustrator and Adobe InDesign is a good starting point. It is fairly straightforward and highly recommended.
The simplest (and cheapest) approach is to play with PDF Transparency is to use vdptech Transparency Viewer. You can freely download the latest version from www.vdptech.com. Here is a snapshot of it: