IMessage doesn't say delivered like it usually does, what does this mean?

Or their phone is turned off.

Is it blue or green?

I use my iPad so it’s blue.

So that means that they had their phone off. If you used your phone, and it didn’t send as an imessage, it would have gone back to a standard SMS text. But since you used your ipad, it doesn’t have SMS, and it froze.

There’s an option in iMessage to turn off sending read receipts so it may mean the person has selected this option. The message is still delivered but their phone won’t tell server.