Help me remember the name of a book I read as a child, please!

[HumpHumpWhale]

I'm sorry this is so vague, but it's been driving me nuts, and I can't work it out alone.
It was about a girl who went to live or stay in the countryside, maybe moors? and ended up in a parallel world or dimension. I remember there being a big house, and something to do with balloons, maybe a hot air balloon? And it was quite sinister. It was kind of similar in feel to Moondial, but I think a bit darker. I think in the end she made it home, maybe through a mirror, or possibly through a rip in the sky.

Writing it down, it really doesn't seem like much. But maybe someone will have a idea!

Thanks!

@HumpHumpWhale When would this have been? 1980s? 1990s?

It's not Blackhearts in Battersea is it? That has a hot air balloon (on the cover and in the story.) It's bloody brilliant. One of my favourite books of childhood. It's the sequel to Wolves of Willoughby Chase which is about a girl going to stay in a big house on a wild moor. Maybe you had a copy of both books in one volume?

Or possibly Midnight Blue by Pauline Fisk

I posted this and then forgot to check back - I think it's midnight blue, thanks SO much, that's been bothering me on and off for years!