Magic speaks quietly now
He noticed the change immediately. He'd been spinning when she'd bounded in, watching the gold swirl 'round and wondering when his beloved caretaker would return. She opened the doors with a flourish, a forgotten plume of smoke abandoning its post at her feet. She skipped across the room, grabbing his shoulder to haul him from his stool and tightly embrace him. Before he had the chance to return the contact she was gone, 'poofing' herself out the room. He could hear her waltzing about outside, and knew immediately that she had found a way to free herself.
Of course, a sorcerer of any kind must always try to ignore the seductive voice of magic when it calls to him, and for anybody it can be near impossible to prevent yourself from falling into the habit of dark magic. It was just so much easier to use. But the Dark One found ignoring these cries much more difficult; the curse itself wanted power, wanted to remain, and that meant no weaknesses allowed. There was no helping others, no free deals, just the darkness. If Belle had managed to change her magic's path to light magic (meaning what was considered 'pure magic' (such as Fairy Dust) would no longer react badly to her), she could now use her magic as she wished. She would still hear the call of dark magic but it would be a whisper not a scream, and much more easily forced to the back of the mind.
And since she was not morally corrupt, and had no one about to do the corrupting, and had no wishes for power or revenge (except against Regina, but her downfall was a current work-in-process), Belle would probably manage to keep her hidden darkness in check for a long time. And now that Belle was herself again, joyful and skipping around the castle in a daze, Rumplestiltskin couldn't be happier. Except if she still loved him. That would certainly be nice. If only he hadn't cursed her.
And didn't plan to do so again.