How do I send blackberry and other users a text message with my iPad?

You can't with iMessage, just as Blackberry users can't make use of Blackberry Messenger with non-Blackberrys - not unless RIM provides a Blackberry Messenger app for iOS devices.

With an iPhone you can make use of an app such as WhatsApp which is cross platform - there is an iPhone, Android, and Blackberry version of WhatsApp, but there is not a WhatsApp version for the iPad. WhatsApp makes use of the cell phone number with a cell phone.

You would have to download a third party app. I would recommend textfree, it will give you a number to text from on your iPad to text people on a blackberry or any phone. Or you could get ping chat, it can text anyone with an iPad, iPod touch, iPhone, android, blackberry, or other smartphone with ping chat installed.

WhatsApp is not going to work on the iPod.