How Celestia would have reached the conclusion that Luna wanted eternal night is understandable: Luna raised the moon to completely eclipse the sun in the middle of the day. At this point, though, Luna was still after the appreciation of her subjects, so her plan may have been to raise the moon then to try and reverse her subjects' sleep schedules. After she took over Equestria, she could create a nocturnal paradise with them and have her night appreciated while still providing the sunlight the planet needed to survive. If this had happened the way she'd intended, Luna may have been able to convince Equestria that it was her sister who had gone evil and that she'd had no choice but to kill her to protect the kingdom (which she believed anyway). She could also have easily explained that her new form was necessary to gain the power to defeat Celestia. Soon, Queen Nightmare Moon would have become the benevolent ruler that Celestia is by the beginning of Season 1, except that Equestria would have been completely nocturnal.
EDIT: after seeing the Season 5 Finale, I am proud to say... ha! Nightmare Moon's eternal night would have TOTALLY WORKED!! They don't need sunlight! Fools, fools all of you, who thought that she would have murdered everything with her plan! No longer does Nightmare belong on the very bottom of Lily Peet's "villains" list!