Chat apps for iPhone and Blackberry?

My cousin and I are so close and we always talk through BBM. I just got an iPhone so I'm not sure what app we could use to still talk. We were going to use Whatsapp but its not working on her phone and needs to get it fixed. Could you please name any apps we could use in the meantime? I can't text because I live in London and she lives in the States. :\

Kik messenger is a very popular app that a lot of people use.

There's Kik which is popular, Live Profile is also good. You can also get Touch (formally PingChat) and there are plenty of others. Does she have Facebook? Facebook Chat, MSN chat… The list goes on

Msn
its free on both phones
and you just have to download it sign up and add her
and you would be able to send unlimited free messages to her
hope i helped.