I guess it depends on whether that Underworld portal required specifically Dark One magic or just magic/dark magic and the blood of someone who'd been revived. If it was just magic, Emma would be able to open it (with Rumple's blood) and if it was dark magic Regina could (again with Rumple's blood). Was this ever clarified?

I was just really irritated that they'd had Rumple making a villain's decision AGAIN and lying to Belle/hiding it from her AGAIN. By this point it'd gone from being surprising plot twists to predictable and annoying. I'd have preferred it if they'd explored non-Dark-One Rumple for longer.