iOS 8 & 7, iPhone 5S
I have one phone number my iPhone insists ought to be iMessage-capable. The system is actually an automated response-counter I need for work to confirm I have received job details, & has never been associated with any Apple device.
I don't know that device's specifics & probably will never be able to find that information, so all I have to go on is "it never was a number associated with any Apple device or service".
At least in iOS 7 I could switch to Airplane mode, send my text, wait for the immediate error, then switch off Airplane mode, Send Again & it would go as 'green' SMS. In iOS 8 this no longer happens - instead it sends again as 'blue' iMessage which never arrives.
So, the question, to which I'm sure the answer is "can't be done" is
How do I stop my iPhone sending iMessages to that number?
Though nothing appears to have changed in my setup, in recent weeks [perhaps since iOS 8.1.1], though these messages still send "in blue" they are actually now successfully arriving at their intended destination.
Very late edit:
The reason it now works in blue is because they now actually have an iCloud account associated with the number.