text bubbles suddenly changed from green to blue despite no change in devices

I was texting with a friend not using an iphone device, and the bubbles were green. I am aware that this is the SMS platform and that seemed perfectly logical given my friend does not have an apple device. Suddenly, however, the text bubbles I sent turned blue, even though I know that they do not have an iPhone. Does this happen when the non-Apple device blocks your number? I've found 1000 different versions of this question, but none that answer this specific issue. Given the situation above, my question is: Does the change from a green SMS bubble (on an iphone communicating with a non-iphone) change from green too blue when a number is blocked or is this a glitch? under what circumstances would something like this happen?

Thanks.