how do i get a hydrus blob?

if you already have a hydrus pet you can transform them to a blob via blob land in darkside which will make them a hyrdus blob, otherwise you will need a blob from blob land and then wait for merana to come and do her quest to transform a pet to hydrus colour.

https://subeta.net/explore/blobland.php

when does she appear? how do i find her?

I believe there is a ping group you can join that alerts when Merana is available - Merana-spotting

thank you

To fully answer your earlier question tho - she will appear on the Explore Subeta page (the one with the globe) and will be there for about 12 hours. It's random when she shows up though, so thats why the ping group is useful.