In order for a message to get sent as an iMessage, both parties need to be logged into iMessage and both parties need to have an active data connection, either WiFi or cellular. It's that second part that's probably the issue here. If one or the other of you doesn't, at the time the message is sent, don't have an active data connection, the message will be sent as an SMS (if you've got that feature turned on). Once the data connection is re-established, you'll go back to iMessage.
I have a friend who sometimes texts me when they are on their way home from work on public transit. Part of the route is underground where there is enough of a signal for cellular voice/sms but not anything else. So, when they are underground, our messages turn green. When he gets out of the tunnel, they go back to blue.
Although I've heard people say that if you block someone, iMessage turn to green, I've never seen any actual documentation from Apple on this. And, someone who "accuses" you of blocking them would be someone I'd seriously consider actually blocking.