Old iMessages are changing colors? In Messages I have a thread of sent messages that showed up as blue at the time of sending. These old texts remained blue until about 15 minutes ago; now they’re green. 

I know sending the messages didn't fail because the conversation was successful. I don't know why, now, the message color changed from blue to green.  Does this mean I've been blocked?

I searched for answers on Google but didn't find anything that helped.
Blue means the contact is using the Message app on an Apple device (either desktop or mobile). Green means the contact is only available via traditional cellular SMS messaging protocol (i.e. not using an Apple device).