Here's something I discovered.
TO FADE A pdf XREF.
In model space, click on the boundary line that surrounds the xref.
When you do that a heretofore unseen tab magically appear--it's called PDF underlay at the end of your ribbon. It's very helpful--which is why the AutoCAD folks don't talk about it or make it known.
Anyway, a new set of options will appear with that PDF UNDERLAY tab--they appear below the ribbon: ADJUST, CLIPPING, OPTIONS, PDF LAYERS, TOUCH
I have no idea what they hell they all mean, but for the purposes of fading the pdf xref, click the ADJUST button. This will bring up a new window with a very sensible Contrast slide bar and a FADE slide bar. You can slide the bar and instantly the image will fade as you move it. Why this magical window doesn't appear when you type Xdwgfadectl is a mystery only AutoCAD can explain.