Messages to an iPhone

You weren’t blocked. Blocking would not change what you see in any way. Its completely transparent to the person sending the messages for obvious reasons.

Its likely if they are on vacation, they are in a place where they don’t have an actual internet connection or they don’t have a data roaming plan if out of the country,, but get enough cell service to get SMS messages.

If the person you are trying to send a message to is also an iPhone user, it should send as an iMessage. The only time it wouldn't is if one of you has a problem with iMessage. iMessage will only work if both parties are signed into iMessage and it is activated. It also requires that both of you are connected to a stable internet connection, either wi-fi or cellular. If one of these steps is not there, it will be an SMS. It could be you or the other person, so check with them to make sure they are signed into iMessage and have a stable internet connection, and check the same for you.

2 possibilites:

1. They aren’t connected to the internet, so cannot receive iMessages.
2. They have not activated iMessage, or have turned it off and as such can only receive SMS text messages.

They will stay green. As they have been delivered as an SMS. They will not change the method of delivery after the fact.