Just a follow-up... although I still have not cured the issue of mail messages being "tagged" as blue folders instead of email envelopes, I learned a way (from Blackberry tech support site - KB10167) to delete any message via the Desktop Manager:
Warning: Be sure to back up data on the BlackBerry device before clearing any database. Information will be permanently lost otherwise.
To clear the Messages database, complete one of the following options:
To clear the Messages database using BlackBerry® Desktop Manager, complete the following steps:
Connect the BlackBerry device to the computer.
Open BlackBerry Desktop Manager.
Double-click Backup and Restore, then click Advanced.
From the list of Device Databases, select the appropriate database. For Email Messages, select Messages.
In the Warning window, click OK.
Send a new message to the BlackBerry device. New messages now appear in the Messages screen.