High Speed Sync vs a Neutral Density Filter Bernie
On a bright day, typical exposure settings for off camera flash might be 1/250th at f8 or f11, ISO 100, so at your maximum sync speed it's impossible under normal circumstances to shoot with wide apertures in order to blur the background or do selective focusing.
There a couple of solutions, neither is perfect, but they do work. you can use either High Speed Sync (HSS) or a Neutral Density filter. There are pros and cons of each and this video explains them, I also carry out a simple experiment to compare the two solutions.