This question just came up on another forum, I have bred DF Yellow/Blue x Silver and got gorgeous, large, healthy birds.
But - you need to check out Ebichua's reply to the blue back to blue back question in the species section:
http://finchforum.com/viewtopic.php?f=48&t=15571
This hits the nail on the head and goes into more detail than what I would normally say of just understanding the birds that are being bred and what their background is.
I never had problems breeding DF Yellow x Yellow, other breeders have said they've had problems, perhaps it's the stock they were working with.
Similarly, many breeders have had problems breeding blue x blue and recommend breeding with splits.
I still do not feel comfortable breeding blues together, but think breeding blue x /blue (including yellow/blue x silver) is fine - but again, take into consideration what he is saying: the quality of the birds themselves must be addressed as well.
It's not about the mutation so much first and foremost as the breeding quality of the bird.