BlueMail marks emails as read or unread using a status indicator: an empty circle means read, and a filled one means unread. The circle's color identifies the account, which helps you prioritize in the Unified Inbox.
Unread mail is also picked out in bold, and how much of it is bold is up to you. Unread indication under Settings | Global Settings | Customize Appearance offers Sender, Sender & Subject or None; the default is Sender, so the subject only turns bold if you choose Sender & Subject. See Unread Indication.
You can change the Read / Unread status of a message.
