Blocs does a lot of things but it can’t do everything. It’s possible to write browser/OS specific CSS but its not something I’d add controls for in Blocs.
Before I continue, I should mention it’s possible to change anything with Blocs by writing your own CSS code tweaks and attaching the additional style sheet to a page in the page settings.
I’m not sure I fully understand the issue, in Blocs parallax is switched off on the Screen dimantions of an iPad, and below the 768 breakpoint. Maybe something in your site (an additional style etc) is preventing this or maybe your iPad screen size is different from the one targeted in the Blocs code.
When parallax switches off on mobile/tablet you get a standard background effect. Are you not getting that, I can’t tell from your screen shot?
If you can share the following I’ll do some tests:
iPad model( screen size)
Live URL with issue
Notes on what section is problematic.
If this is a weakness in Blocs, let’s work together to make Blocs even better!