I had a blue spot for a couple months before I did something stupid and killed her. I was experimenting with vodka dosing to manage nitrates and got a bacterial bloom. When I discovered the problem, the ray was already struggling. All other fish inhabitants were unaffected. From this, I hypothesize that these rays are more oxygen sensitive than other fish. I also think that they may suffer more from gill burn during shipping than other animals. Perhaps this might help partially explain why they can be so hard to acclimatize.
For feeding, my ray loved PE mysis. I would turn off the circulation in the tank and use a baster to put down a pile of shrimp. She would "pounce" on the pile, and form a dome over the top of it while she ate. It was pretty cool. Many people suggested squid, but I never had good luck with it. The squid I had was very tough stuff, and I don't believe these rays have any sharp teeth, only bony plates. I'm not sure how they would break down something as tough as squid. Perhaps if it was cut down into very small pieces it may work.
Last I found the ray really liked having a shaded overhang to hang out under. She would swim around quite a bit, but her favorite spot was in the shade.