I just tried by putting a navigation bloc in the top global area and set the background to parallax and it works just fine. It maybe that you’ve set the navigation block to be sticky, this will cause a problem. One way to solve this is to use a hero bloc that contains the navigation. You can then set the background of the hero to parallax. This will cause the navigation to become white when it comes into view, but you can change the colour to transparent, or semi transparent. Just select the hero block and set the background colour to be transparent. Do this by selecting the background colour in the right properties panel. Create a swatch and set it to any colour you like, then adjust the opacity slider to make it transparent.
If you also want to get rid of the bottom shadow on the sticky nav bar, open the class manager and add a class called .sticky-nav.sticky. Then edit the class and select the shadow tab. Choose box shadow and again, reduce the opacity of the shadow colour:
Here is an example bloc file.
navigationBG.bloc (1.4 MB)