I've been looking around but I can't find an answer on this. I'm trying to make a view that will display it's contents normally, but anything that's underneath it (in the z axis) would be blurred.
I can't seem to manage to do this right off the bat. Anything I try (at best) blurs the contents of the view, not what's underneath.
Check out this article on Cocoanetics, it gives you some instructions how to properly set up a NSView with blurred background: http://www.cocoanetics.com/2013/10/blurring-views-on-mac/
There's also a RMBlurredView class on Github: https://github.com/raffael/RMBlurredView
The idea behind it is to use a layer-backed NSView with a CIGaussianBlur CIFilter applied on the backgroundFilters property. All important flags are set correctly in the mentioned class.
If you use an image that is significantly smaller than the view's frame and scale the image up, it will be blurred.
You could use CALayers. Declare a root layer and add the background image as a sublayer. Put the "contents" in a subview with a transparent background. (Otherwise the "contents" would probably be obscured by the blurred sublayer.)
Here's partial (and untested) code for setting up the sublayer:
// Set up the root layer.
[[self.aViewController view] setLayer:[CALayer layer]];
[[self.aViewController view] setWantsLayer:YES];
// Set up a sublayer.
CALayer *blurredSublayer = [CALayer layer];
NSRect rectOfView = [self.aViewController view] frame];
blurredSublayer.bounds = rectOfView;
// Views are usually positioned from their lower left corner, but CALayers are positioned from their center point.
blurredSublayer.position = CGPointMake(rectOfView.size.width/2, rectOfView.size.height/2);
// Set the sublayer's contents property to the image you've chosen. You might need to do the scaling when you set up the CGImageRef used by the contents property. (I haven't done this; I leave it to you.)
// Add the sublayer to the view's root layer.
[self.aViewController.view.layer addSublayer:blurredSublayer];
// You'll probably want to save expense by calling setWantsLayer:NO for the contents-bearing subview, since it will have been turned on when you set up the superview's root layer.
Or it might be easier to use a CGContext. But with the CALayer you have the the zPosition property you mentioned.
P.S. The use of "contents" and "superview" and "sublayer" make the structure confusing. Here's a descriptive hierarchy. The first-named item is on the bottom and the last-named on top, as it would be in IB:
