BoltMail
If a contact signs up to multiple contact lists using the one form and one of the lists is hidden is it possible to only tell them about the one list?
Yes. You simply need to change what the conformation emails/HTML and thank you information says.

The contact will only see the one form and subsequent pages/emails and so if you set these to only tell them that they have signed up to the one contact list then they will only be told that.