FYI. YMMV. For GamePass PC users..
In order to successfully log into the Xbox (beta) app, and not get a black app screen after going through the sign-in, you need to go and log into the Xbox Console Companion app first. Doing so will give you a Windows 10 notification saying you are signed into Xbox Live, and from there you can go launch the Xbox (beta) app, sign in, and not end up on the black app screen after. This was what I discovered worked for me.
Tried DNS setting change on my router, after one guy commented that helped his issue with the beta app, but that didn't make a difference for me.
That said, you still have to change your Windows 10 APPS default storage location setting... if you do not want your games installing to your OS (C:) drive. Which is dumb - hopefully they fix that and let you pick location on the fly during installs.