While on a phone call my screen blacked out and I could not make any selections. The screen finally showed up but not right away. Does anyone know why?

Perhaps you inadvertedly obstructed the proximity sensor (at the top, close to the front camera), or it isn't working properly.

If the issue persists, for your peace of mind, I suggest that you make an appointment (*) and have your device inspected at an Apple Store or AASP (Apple Authorized Service Provider) among this list:--> Apple – Location Selector. After checking it out, they'll give you all the available options.

Regards

Giulio

(*) You can do it:

• Online:--> Genius Bar Reservation and Apple Support Options - Apple;
• Contacting a local Apple Store or AASP among the list provided above;
• Using the "Apple Store" app, available and downloadable from AppStore.

My pleasure