You can just use string.Contains to see if the agent has 'Blackberry' within it
string agent = Request.UserAgent;
This should tell you that the client is using a blackberry device, but nothing more specific than that. Is that what you wanted?
Edit after Comment:
Possibly look into the 51degrees mobile device detection project which uses the WURFL device database
This should allow you to determine device models without having to add lots of string parsing