What does blue text message mean?

If the messages (and the Send button) are blue, you're chatting over iMessage with another iOS 5 user.

If they are green, you are using standard SMS text messaging.

Your device will automatically detect this and send whichever way is appropriate.

Blue means the message was sent as an iMessage. See p. 66 of the manual... http://manuals.info.apple.com/en_US/iphone_user_guide.pdf