Thanks for replying Jenny

The Yorkie I talk about was my OH's and his Ex wife's. He was an adult rescue and they were his last chance because of his agression, so the cause of his aggression wasn't known. However a little research by ex-wife found some dog's he was related to also had poor temperaments. Hubby's thoughts are that the dog was bred for colour to the detriment of temperament, but he has no real evidence for that. Having not met a Blue myself, seeing your post made me curious about Blue temperaments in general.

Your boy sounds an absolute sweety though As mentioned by Dundee, you do run a very real risk of his temperament changing if you let him mate so you need to bare that in mind