This happens all the time.
When you catch the clean your chest needs to be puffed out and your shoulders need to shrug forward. This will make a little shelf that the bar rests on. Your elbows should be pointing straight in front of you. If your elbows point down or at an angle towards the ground, then the bar will be supported by your arms on on your clavicles, which why you're blacking out. If you catch the bar right you can hold it and breathe normally.
Here's an example of poor form (elbows point down and bar is supported by her arms and neck):
And here's good form (very solid "shelf", bar supported by the shoulders):
A great way to warm up is to load the bar with a heavy weight while it's on squat racks. Get into position like you will be front squatting it, but keep it on the rack. Move your elbows forward and back like you're cleaning it, gradually limbering up the shoulder joint until you can point them straight forward. It's a mobility drill, do it until you feel limbered up. You can also have a partner push up on your elbows to stretch you into that position, but the partner stretch is best done after a workout.
Best of luck!