Capture One can only soften or de-emphasize existing detail; it doesn’t provide true synthetic lens blur/depth-of-field simulation. Using a local adjustment layer with reduced Clarity/Structure/Sharpness can give a mild effect, and some users also try a smart brush with negative Dehaze on a fairly uniform background, but these are limited and won’t convincingly mimic real out-of-focus blur.

For a strong, believable shallow-depth-of-field look, you’ll generally need to round-trip the image to an external editor or plugin that offers lens blur/bokeh tools.