Hey JS29,
Welcome to Crowd Support 
How was it that you were accessing your emails, was this online via the Webmail service or do you use an email management program to send and receive emails?
The FAQ's that we have for Telstra and BigPond Mail can be located at:
https://www.telstra.com.au/support/category/email
If this is accessed online then all un-opened mail will still be dark, as opened mail will show as being lighter to indicate that they have been read.