It was the Curse To End All Curses, to take them to a land without magic. If the Blue Fairy couldn’t figure it out, just from that alone, the girl needs to find a new profession.

But Blue didn't know that (from what we saw).

Here's the exact text of what the Queen said - and I don't see a way to determine the nature of the curse (or even that there'd be a curse) from it: But you shall have it! My gift to you is this happy, happy day. But tomorrow, my real work begins. You’ve made your vows, now I make mine. Soon, everything you love, everything all of you love, will be taken from you. Forever. And out of your suffering, will rise my victory. I shall destroy your happiness, if it is the last thing I do.

All she said - is "I'm gonna take away what you love and make you suffer" - I personally do not think that is anywhere near specific enough to know it's about sending them to a world without magic, trapping them in time, or has a thing to do with Rumpelstiltskin. It's actually pretty vague and could be bandied about by a number of villains.

They only learned about the other stuff *after* speaking to Rumpelstiltskin, at which point they discovered the specifics, including the only cure.