How is the blurring of the preview view realized?

Scripting
2

That is a CoreImage filter on the view/layer.
here is some code that might help:

CGFloat radius = 30;
_blurFilter = [CIFilter filterWithName:@"CIGaussianBlur"];
[_blurFilter setDefaults];
[_blurFilter setValue:@(radius) forKey:kCIInputRadiusKey];
if (@available(macOS 10.14, *)) {
	view.layer.filters = @[_blurFilter];
}
else {
	view.contentFilters = @[_blurFilter];
}