2) Portraits. Everyone talks about using a small f/stop >=2.8 to produce background blur (bokeh) and give the shot a nice affect. However I find - that f stops 2.8 and smaller - can easily produce blurry shots as the smallest movement of the subject (not to mention a group shot where people are not all the same distance from you) causes blur. I found that nothing ruins a nice portrait more then a blurry kind of picture (unless this was intended for some artistic purpose)
What is the best F/stop for portraits ? What about if you use a flash ?
2.8 and smaller, open up the aperture, allowing a faster shutter, thereby allowing you to freeze your subjects better, so in this case, it's a win/win, since you get more softer backgrounds anyway. The 2 downsides (minor) are that some lenses will be sharper 1-2 stops slower, and if you are up very close, the depth of field will be so shallow that unless your subjects are in one plane, some will be blurred.
Perhaps you are referring to OOF (out of focus) and not (motion) blur caused by someone moving front to back and falling outside the depth of field... ?
To get max background blur, open up as much as you can knowing your lens will still be sharp. In the end there are many factors to consider but for this scenario, open up the aperture and work from there. Know your lens!