Lagoon Blue over pink?

It will depend on how much orange is in the peachy pink. Orange and blue make brown, so you could get a muddy brownish colour, or if the blue is very diluted a blonde shade.
If the peach is more pink than orange I think you will get a muted lilac. I haven't used lagoon blue since they reformulated the colours, but I always found it quite weakly pigmented, so i doubt it will cover. Lilac shades are more likely.

It's definitely more pink, there's only a few streaks that are peachy pink.

edit: it's pretty much like my avatar, just more faded now.

Try on a strand if you already have the dye, I think it will be a pretty colour as long as the orange tones are minimal