Why doesn't iphone6plus turn blue when imessage is turned on?

You sent it to a person who also uses Apple, and that person also turned on imessage. Are you sure he logged in with his mobile phone number or APPID?

If the person you sent activated imessage with his mobile phone number, then you send him a message, and he is in a place with Internet (that is, connected to wireless or turned on cellular data), then the SMS will automatically switch to imessage.

If the other party is activated with an AppleID, you should enter his AppleID instead of his number when sending a message, and he needs to be in a place where there is Internet (that is, wireless connection or cellular data is turned on), then the SMS will automatically switch to imessage.

If he is not an Apple user at all, or if he doesn't turn on imessage, or if he is below IOS5.0, he must send a green message.

Note: Green stands for SMS. Blue represents imessage information.