Found the problem! Blown fuse in the Battery Tender harness that I was trying to use!
When connected to the motorcycle, the Battery Tender connects to a harness that I had pre-wired onto the battery terminals. That harness allows me to simply plug in the Battery tender harness to the harness attached to the battery, quickly and simply without gaining access to the hidden battery. It also enables me to power a GPS on the motorcycle's handelbar from the battery, or a heated riding vest. That harness has an inline fuse.
To connect to the Camaro's front terminals, I used a CLAMP type harness, also fused, that I had only ever used once before. But that "once before" was to try to charge a battery that I knew was very weak. The charging attempt failed, but it failed, I now see, because that old motorcycle battery was SO weak that it tried to pull more amps than the 7.5 amp fuse in the clamp harness allowed (because the battery wasinternally shorted?). So, it had blown the fuse. I never realized that it had done so, and so the blown fuse was still in the harness.
The amber blinking light on the Battery Tender begins to blink when EITHER a connected battery is so bad that it won't accept a charge, OR when there is simply no connection. So, in the case of the too-weak motorcycle battery, it had correctly begun blinking because the battery was bad. In the case of the Camaro attempt to charge through a blown fuse, it began blinking because there was no valid connection because of the blown fuse. Two different problems created the same visual signal.
I have a 2nd clamp harness that I have never before used, and so I connected this 2nd clamp harness to the Camaro. Now the Battery tender is happily doing its job.
Jim G