Chat in Yahoo Messenger using your blackberry smart phone might be not hard enough. All you have to do just install the Yahoo Messenger for Blackberry as your YM Chat Client and then login from its.
Yahoo Messenger for Blackberry
You can have all in one mobile messenger such as Xumii to chat in Yahoo Messenger, or you just install the stand alone ym for blackberry.
Features in YM for Blackberry
Status Message, IM Windows, Emoticons, Contact List and many more
Free Download BlackBerry Yahoo Messenger Chat Client
There are two main source that you can download YM for Blackberry: from Blackberry official website and from Yahoo Messenger Official Website. It freeware /adware that why you can use and download it free.