How to blur screen shots for iPhone?

So I have an iPhone 4s and I see people post pictures on Instagram of a screen shotted conversation but they blur some of the text out and focus on one section. How do I do that?

Take a screen shot, then transfer the picture to your computer.
Use your computer (with an appropriate program such as Photoshop or PS Elements or Gimp or PaintShopPro etc) to blur parts.