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?
Late Edit:
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.
Best Answer
See if you can opt that SMS number out of iMessage entirely at https://selfsolve.apple.com/deregister-imessage as explained in more detail at How do I disable iMessage for a contact who no longer has an iPhone
If that doesn't work, contacting AppleCare to enlist the support of an engineer might be needed. In the short term, you can disable iMessage entirely and SMS functionality should be restored for all SMS eligible costacts, but you lose iMessage for everything else as well.