Carl, I believe the Garmin 250C dual frequency replaced the 160C Blue. I've used both and couldn't really tell a difference. I think the dual frequency is supposed to help out in deeper waters....you can get the color 250C DF for about the same price - probably the way to go....btw I have the 250 black and white and its great.
Here is a link:
http://cgi.ebay.com/GARMIN-Fishfinde...QQcmdZViewItem
Blue