Azorah
To answer your questions:
"can the blueyonder account still be used?"
The Blueyonder email account will remain unaffected. He will still be able to send and receive emails with his blueyonder.co.uk address.
why do we need the gmail account ?
VM introduced the need to assign a separate non Virgin Media email address to every VM email account to improve the security of VM accounts. It does not have to be a Gmail account, just an email account from another email provider. It could be an Outlook, Yahoo, iCloud account or any-other email account not provided by VM.
That non VM email account is the email address where VM will send password reset verification emails when you change your VM password. That means it is now more difficult for hackers to change your password and hijack your VM email account without your knowledge.
The non VM address will also become the username for your VM account when you sign into your VM webmail account and My Virgin Media account.
So when sign in to your Blueyonder account you will need to type the Gmail address as the Username and then the password of your Blueyonder email account. That will take you to your Blueyonder account - not your Gmail account.
Again, that makes it more difficult for hackers to hack into your VM email and My Virgin Media account. In the past they could be pretty certain that the username for a VM account was the same email address. Now they will need to know your non VM emall address as well as your VM address.
VM could have used a different way of achieving that level of additional security but that's how they chose to do it.
"think ive messed up access to my husbands blueyonder address. "
That may be because you are still trying to access his account using his Blueyonder address as the username. As I said earlier, he now needs to type his Gmail address in the username field when accessing his Bluyonder account on the VM website.
Finally, post back if you use an email app or client to access your Blueyonder emails as well or instead of the VM webmail service. There is now another change needed to do that but I have not mentioned it here to avoid over complicating matters further.
Coenoby