Blorb files are normally associated with Inform, which both Gargoyle and Zoom should be able to run.
However, I'm guessing that you're talking about Working Man and Let Me In, which are both ADRIFT 5 games that were packaged as blorb files. Since you can't use the exe versions of those games, you'll have to install ADRIFT 5 (which is available from http://www.adrift.co/download). The ADRIFT runner is supposed to work on Macs, but not being a Mac user I've never tested that.
Hope that helps.
IIRC, both Glulxe and Git can run gblorb files (assuming you're using Windows). They can be downloaded from here. Git is effectively a faster version of Glulxe
Edit: There's actually a useful table on the wiki that lists available programs to run specific file types
I can't make an equivalent to the executable file for Mac, unfortunately. I have to only offer the same options I usually do for Mac (the blorb file, etc). You can get that in the zipped version of the game here: https://www.patreon.com/posts/let-me-in-v1-0-0-2748012
I have actually been distributing test builds of stuff on itch.io - if you create a project and set it to "Draft — Only those who can edit the project can view the page" it offers you a "secret URL" at the top of the page that you can give to people to download and test the game privately, or play it directly there if it's web-playable (they won't be able to edit the game page via the secret URL unless you grant them that permission). The project also won't be shown publicly or searchable on the main site or web search (just don't accidentally set the status to "Published".) You can also require a password if you want, or generate keys for testers or early access. A project can also be completely deleted if you don't intend to distribute via itch.io.