why, sometimes i send imessage to another iphone its a green bubble

A Green bubble means its being sent as an SMS. This can happen if either you or the recipient do not have an internet connection, either Wifi or Cell Data when you attempt to send the message. Blue iMessages require an active internet connection on both ends to be sent.

About iMessage and SMS/MMS - Apple Support

If they just got the iPhone, it's possible that it hasn't registered yet. This should go away over time. It's also possible that they aren't logged into or don't have iMessage set up properly.

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iMessages should stay blue even if you are blocked.

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