Custom actions not accurate; also, atn files not in folder

No, a flat image is not a requirement, but a Background layer may be necessary. My guess is that when your file lacks a Background layer, the canvas size is expanded to include transparent areas. It seems like you have black color filling the areas outside the visible canvas, which is why you see a black expansion. Try deleting everything outside the visible areas before running the action by selecting all and then using Image > Crop. After that, run the action, and it should result in a transparent expanded area.

And finally, and perhaps a key, if I flatten before running canvas expansion, it results correctly with gray canvas extension.

Didn't think to try this before. Don't know why that makes adifference, but it's a solution.

I've added a black bg layer for additional testing. When I run the action (resize, and then expand canvas) with the black bg layer, I get transparent extensions, when I turn that off and run with original white bg I get gray extensions (that's correct and expected) and with black color fill layer only, i get black extensions instead of gray, as reported above. This is with a newly created action, saved to the presets folder. Thanks for your ideas. This screenshot is the original psd before running any action.

Please post a screenshot with image and layers panel showing.

You save actions in the Actions panel hamburger menu.

https://helpx.adobe.com/photoshop/using/playing-actions.html#playing_and_managing_actions

Deleted; duplicate reply

I've always used the canvas size dialog to set the color. Regardless of my choice, i get black as a result. This is unexpected behavior.

This is not a large file and my available disk space is over 100GB. 

The canvas extension color is set in Canvas Size dialog.

The scratch disk full message means you're very low on disk space. Image editing moves huge amounts of data, much more than any RAM you may have installed. So temporary working data are written to disk, aka the scratch disk. If you get this message, you need to clear out space or get a new/bigger drive.

Actions are stored as temporary items in your general preferences until you save them out for safekeeping. You can save them anywhere you want.