Believe it or not, I have seen a large number of well-meaning New Yorkers who try not to step into a photographer's photo. They'll stop dead in their tracks, and politely wait for the person to capture their image before continuing along. I commend these fine citizens for their courtesy, but I do have a somewhat odd request. Please, feel free to step in front of my lens. Yes, I really said that. Allow me to explain.
When someone stands in front of your subject, it unlocks a rich world of compositional layers. This added depth leads the viewer's eye from the foreground through the middle of the scene, all the way to the background. I believe the relationship between these different aspects of a photo can work together to tell a story.
While some of these moments are created through what Bob Ross referred to as "happy accidents", the observant photographer can work to knowingly incorporate this concept into their repertoire. As you'll see, it can even be used to eliminate distracting elements and focus attention on your subject.
As for the technical details, the level of blurring is dependent on three things;
A wide aperture like f2.8, or f4 to create a very shallow depth of field.
The distance between your lens, and the subject you're trying to blur. The nearer they are, the softer they'll be. This is a result of the lenses minimum focusing distance. For example, with a Canon 70-200mm lens, I need to be at least 4.9 feet away in order to achieve focus. Anything closer than that minimum distance will be out of focus.
A telephoto lens like a 70-200mm will enhance this effect.
The next time someone steps in front of your lens, don't let off your shutter button. Keep shooting, and use their presence to create a unique image.