Well, that's what doesn't work, unless I sand off the finish on the inside surface. That is a bit difficult to do well, unless I open the "u" further (it's closer to a "C", really), sand it, then bend it back to the shape I need. The pieces are small enough to be a bit hard to hold on to while sanding. Possibly a small wire brush attachment on my Dremel tool would work better than trying to sand by hand. But, I'm just puzzled / curious as to the problem. I've done a lot of copper pipe soldering in the past, but that was to the outside surface of the pipe or tube, slipping into some fitting. Never had a problem with the inside surface of a fitting, if new. Anodizing the inside surface of the tube WOULD sort of make sense to extend its lifetime. Or maybe this is oxidation, but not to the point of darkening or "greening" the copper? This looks very "even" though - not "patchy" at all.
If you can get nitric acid a 25% solution will clean it very quickly but be sure to rinse it really well
“I’ve done a lot of copper pipe soldering in the past, but that was to the outside surface of the pipe or tube, slipping into some fitting. Never had a problem with the inside surface of a fitting, if new.”
You mean you didn’t brush or sand the inner surface of the fitting? If so, it is hard to imagine you got a proper joint.
To get a proper solder joint you always use the wire brush or sandpaper on the inside surface of the fitting just as you do the outer surface of the tubing.
And always sand the tubing a bit beyond the depth of the fitting so you get a good meniscus.